The magnetic Rayleigh–Taylor instability is a fundamental MHD instability and recent observations show that this instability develops in the solar prominences. We analyze the observations from Solar Dynamic Observatory/Atmospheric Imaging Assembly of a MRT unstable loop-like prominence. Initially, some small-scale perturbations are developed horizontally and vertically at the prominence-cavity interface. These perturbations are associated with the hot and low dense coronal plasma as compared to the surrounding prominence. The interface supports magneto-thermal convection process, which acts as a buoyancy to launch the hot and low denser plumes (P1 and P2) propagating with the speed of 35–46 km s-1 in the overlying prominence. The self-similar plume formation initially shows the growth of a linear MRT-unstable plume (P1), and thereafter the evolution of a nonlinear single-mode MRT-unstable second plume (P2). A differential emission measure analysis shows that plumes are less denser and hotter than the prominence. We have estimated the observational growth rate for both the plumes as 1.32±0.29×10−3 s−1 and 1.48±0.29×10^−3 s^−1, respectively, which are comparable to the estimated theoretical growth rate (1.95×10^−3 s^−1). Later, these MRT unstable plumes get stabilize via formation of rolled (vortex-like) plasma structures at the prominence-cavity interface in the downfalling plasma. These rolled-plasma structures depict Kelvin-Helmholtz instability, which corresponds to the nonlinear phase of MRT instability. However, even after the full development of MRT instability, the overlying prominence is not erupted. Later, a Rayleigh-Taylor unstable tangled plasma thread is evident in the rising segment of this prominence. This tangled thread is subjected to the compression between eruption site and overlying dense prominence at the interface. This compression initiates strong shear at the prominence-cavity interface and causes Kelvin-Helmholtz vortex-like structures. Due to this shear motion, the plasma downfall is occurred at the right part of the prominence–cavity boundary. It triggers the characteristic KH unstable vortices and MRT-unstable plasma bubbles propagating at different speeds and merging with each other. The shear motion and lateral plasma downfall may initiate hybrid KH-RT instability there.
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We numerically investigate the periodicity and damping of transverse and longitudinal oscillations in a 3D model of a curtain-shaped prominence. We carried out a set of numerical simulations of vertical, transverse and longitudinal oscillations with the high-order finite-difference Pencil Code. We solved the ideal magnetohydrodynamic (MHD) equations for a wide range of parameters, including the width and density of the prominence, and the magnetic field strength (B) of the solar corona. We studied the periodicity and attenuation of the induced oscillations. We found that longitudinal oscillations can be fit with the pendulum model, whose restoring force is the field aligned component of gravity, but other mechanisms such as pressure gradients may contribute to the movement. On the other hand, transverse oscillations are subject to magnetic forces. The analysis of the parametric survey shows, in agreement with observational studies, that the oscillation period (P) increases with the prominence width. For transverse oscillations we obtained that P increases with density and decreases with B. For longitudinal oscillations we also found that P increases with density, but there are no variations with B. The attenuation of transverse oscillations was investigated by analysing the velocity distribution and computing the Alfvén continuum modes. We conclude that resonant absorption is the mean cause. Damping of longitudinal oscillations is due to some kind of shear numerical viscosity.
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During a solar flare, it is believed that reconnection takes place in the corona followed by fast energy transport to the chromosphere. The resulting intense heating strongly disturbs the chromospheric structure and induces complex radiation hydrodynamic effects. Interpreting the physics of the flaring solar atmosphere is one of the most challenging tasks in solar physics. We present a novel deep learning approach, an invertible neural network, to understanding the chromospheric physics of a flaring solar atmosphere via the inversion of observed solar line profiles in Hα and Ca II λ8542. The network is trained using flare simulations from the 1D radiation hydrodynamic code RADYN as the expected atmosphere and line profile. This model is then applied to whole images from an observation of an M1.1 solar flare taken with the Swedish 1 m Solar Telescope/CRisp Imaging SpectroPolarimeter instrument. The inverted atmospheres obtained from observations provide physical information on the electron number density, temperature and bulk velocity flow of the plasma throughout the solar atmosphere ranging in height from 0 to 10 Mm. Our method can invert a 1k x 1k field-of-view in approximately 30 minutes and we show results from the whole image inversions and error calculations on the inversions. Furthermore, we delve into the mammoth task of analysing the wealth of data we have accumulated through these inversions.
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Studying the polarization properties of penumbral microjets that have the shortest durations requires spectropolarimetric observations with the fastest temporal cadence possible and is currently a challenging task. Here, we approach this task using fast-cadence spectropolarimetric measurements of the Ca II 8542 A line made with the CRISP instrument at the Swedish 1 m Solar Telescope. We exploited the diagnosis capabilities of this line to retrieve the magnetic field configuration and its evolution in the upper photosphere and low chromosphere by applying the weak field approximation to its wings and line core wavelengths respectively. We found that the short-lived microjets are associated with a transient perturbation in the photospheric magnetic field and sometimes they show clear but weaker changes in the chromospheric field as well. We will describe the different types of evolution that were identified.
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Solar prominences are typically considered clouds of plasma suspended, bound, and governed within the solar corona solely by their host magnetic field. That said, recent studies have suggested we reconsider the (widely-adopted!) assumption that prominence mass has a negligible role to play in the (in)stability of this host magnetic field. Specifically, such studies have suggested that the contribution of mass to the global equilibrium of quiescent prominences is quite the opposite. Of course, the plethora of observations of the vertical motions within quiescent prominences (typically interpreted as occurrences of the Rayleigh-Taylor instability) have already suggested the importance of mass on smaller scales. The further suggestion that these falling ‘plasmoids’ may then successfully drain from the prominence-hosting magnetic field and subsequently cease to contribute to the global equilibrium is, therefore, of particular interest. We present the latest results of our ongoing study that combines high-resolution ground-based observations (H-α, Ca II 8542, He I 10830) taken at the Dunn Solar Telescope (DST) with multiple inversion methods that involve a varying degree of assumption/complexity. I will briefly present the instrument set-up and resulting observations of an on-disk prominence from 29 May 2017. Each inversion approach and their individual results will then be discussed, before the construction of one possible framework that accounts for all of the observations. I will then finish with details of a tentative detection of an RTI-like structure located beneath the observed prominence.
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The partially-ionised nature of the lower solar atmosphere introduces new and exciting complexities to shock solutions. Here we study numerically the slow-mode shock triggered via a magnetic discontinuity, mimicking the slow-mode shocks that can form as a result of magnetic reconnection. In single-fluid ideal MHD, the slow-mode shock occurs as a discontinuous jump in parameters. However, in the two-fluid partially-ionised plasma, the shock occupies a finite width due to the coupling and decoupling of plasma and neutral species across the shock. It is found that this finite width region allows for shock sub-structure that can affect the overall dynamics of the system. In particular, we find that an intermediate shock can exist where the plasma velocity transitions from super to sub Alfvenic velocities. A key feature of this type of shock is that the magnetic field is reversed across the interface. We present numerical results analysing the formation and evolution of intermediate shocks as sub-structure within slow-mode shocks.
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We present spectropolarimetric analysis of a flux-emerging region (FER) in order to understand its magnetic and kinematic structure. Our spectropolarimetric observations, in the He i 10830Å spectral region, were recorded with the GRIS at the 1.5m aperture GREGOR telescope. A Milne–Eddington-based inversion code was employed to extract the photospheric information of the Si i spectral line, whereas the He i triplet line was analyzed with the Hazel code. The spectropolarimetric analysis of the Si i line reveals a complex magnetic structure near the vicinity of the FER, where a weak (350–600 G) and horizontal magnetic field was observed. In contrast to the photosphere, the analysis of the He i triplet presents a smooth variation of the magnetic field vector (ranging from 100 to 400 G) and velocities across the FER. Moreover, we find supersonic downflows of ∼40 km/s appearing near the foot points of loops connecting two pores of opposite polarity, whereas strong upflows of 22 km/s appear near the apex of the loops. Furthermore, nonforce-free field extrapolations were performed separately at two layers in order to understand the magnetic field topology of the FER. The reconstructed loops using photospheric extrapolations along an arch filament system have a maximum height of ∼10.5 Mm from the solar surface with a foot-point separation of ∼19 Mm, whereas the loops reconstructed using chromospheric extrapolations reach around ∼8.4 Mm above the solar surface with a foot-point separation of ∼16 Mm at the chromospheric height. The magnetic topology in the FER suggests the presence of small-scale loops beneath the large loops. Under suitable conditions, due to magnetic reconnection, these loops can trigger various heating events in the vicinity of the FER.
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ICMEs (Interplanetary Coronal Mass Ejections) are violent phenomena of solar activity that affect large regions of the heliosphere. The prediction of their impact on the Earth and other solar system bodies is one of the primary goals of the planetary space weather forecasting. The travel time of an ICME from the Sun to the Earth can be computed through the Drag-Based Model (DBM). A DBM is based on a simple equation of motion for the ICME defining its acceleration as a=-Γ(v-w)|v-w|, where a and v are the CME acceleration and speed, w is the ambient solar-wind speed and Γ is the so-called drag parameter (Vršnak et al., 2013). To run the codes, forecasters use empirical input values for Γ and w, derived by pre-existent knowledge of solar wind. In the ‘Ensemble’ approaches (Dumbovich et al., 2018; Napoletano et al. 2018), the uncertainty about the actual values of such inputs are rendered by Probability Distribution Functions (PDFs), accounting for their variability and our lack of knowledge. Employing a list of past ICME events, for which initial conditions when leaving the Sun and arrival conditions at the Earth are known, we apply a statistical approach to the DBM to determine a measure of Γ and w for each case. This allows to obtain distributions for the model parameters on an experimental basis and to test whether different conditions of relative velocity to the solar wind influence the value of the drag efficiency. This is a promising approach when considering the extremely short computation time needed by the model to propagate ICMEs, to forecast ICME arrival to planetary bodies or spacecraft in the whole heliosphere, with relevant application to space-mission short-term planning.
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We present the application of the weighted horizontal gradient of magnetic field (WGM) flare prediction method to 3D extrapolated magnetic configurations of flaring solar ARs. The main aim is to identify an optimal height range, if any, in the interface region between the photosphere and lower corona, where the flare onset time prediction capability of WGM is best exploited. The optimal height is where flare prediction, by means of the WGM method, is achieved earlier than at the photospheric level. 3D magnetic structures, based on potential and non-linear force-free field extrapolations, are constructed to study a vertical range from the photosphere up to the low corona with a 45 km step size. We found that applying the WGM method between 1000 and 1800 km above the solar surface would improve the prediction of the flare onset time by around 2-8 hrs. Certain caveats and an outlook for future work along these lines are also discussed.
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Solar flares and eruptions are one of the most energetic phenomena occuring in the solar system. They are typically described by the cartoon-like 2D Standard model of solar flares. This model is however not capable of describing J-shaped (hooked) solar flare ribbons, bright elongated structures typically observed in the UV part of the spectrum. Their description requires 3D MHD modelling of magnetic flux ropes, bundles of twisted field lines rooted in the hooked endings of flare ribbons. The standard flare model in three dimensions, developed in the Observatory of Paris, was recently used to find predictions on how do the field lines reconnect during solar eruptions with respect to the positions of flare ribbons (Aulanier & Dudík 2019, A&A, 621, 72). Authors of this study identified three geometries involving field lines composing and/or surrounding the erupting flux rope. With a help of high-resolution EUV data, these were identified in a series of publications focused on eruptive events. Using data from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory, we will present the manifestations of the different 3D reconnection scenarios and discuss under what conditions can their constituents be observed.
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Propagating kink waves have been reported recently and have been found to be ubiquitous in the solar corona including in the quiet Sun. It is imperative to understand the mechanisms that enable their energy to be transferred to the plasma. Carrying on the legacy of the standing kink waves, mode conversion via resonant absorption is thought to be one of the main mechanisms for damping of these propagating kink waves, and is considered to play a key role in the process of energy transfer. We use the Doppler velocity images of the Coronal Multi-channel Polarimeter (CoMP) for the study of propagating kink waves in quiescent coronal loops. A coherence-based method is used to track the Doppler velocity signal of the waves, enabling an investigation into the spatial evolution of velocity perturbations. To enable accurate estimates of these quantities, the first derivation is provided of a likelihood function suitable for fitting models to the ratio of two power spectra obtained from discrete Fourier transforms. Maximum likelihood estimation is used to fit an exponential damping model to the observed variation in power ratio as a function of frequency. This also confirms earlier indications that propagating kink waves are undergoing frequency-dependent damping. Additionally, it is found that the rate of damping decreases for longer coronal loops that reach higher in the corona. The analysis techniques are used to create a statistical sample of quiescent loops to study the statistical properties of propagating kink waves and compare it to the studies of standing kink waves. It is noted that the damping for the propagating waves appears to be significantly weaker than that found from measurements of standing kink modes. The propagating kink waves also exhibit signatures of power amplification of waves. These propagating kink waves provide a new avenue to perform coronal magneto-seismology even during the quiet Sun period and this reliable method is not limited by requiring the eruptive activity of the Sun.
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The Solar Orbiter mission, whose orbit is outside the Sun-Earth line, opens up novel opportunities for the combined analysis of measurements by solar imagers and spectrometers. For the first time different spectrometers will be located at wide angles with each other allowing 3D spectroscopy in the solar atmosphere. In order to develop a methodology for these opportunities, we make use of the Hinode EUV Imaging Spectrometer (EIS) and Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and by employing solar rotation we simulate the measurements of two spectrometers that have different views of solar corona. The resulting data allows us to apply stereoscopic tie-pointing and triangulation techniques designed for SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) imaging suite on the STEREO (Solar Terrestrial Relations Observatory) spacecraft pair and perform three-dimensional analysis of Doppler shifts of quasi-stationary active region. We present a technique that allows the accurate reconstruction of the 3D velocity vector in plasma flows along open and closed magnetic loops. This technique will be applied to the real situation of two spacecraft at different separations with spectrometers onboard. This will include the Solar Orbiter Spectral Imaging of the Coronal Environment (SPICE), the Solar Orbiter Extreme Ultraviolet Imager (EUI), the Interface Region Imaging Spectrograph (IRIS) and Hinode EIS spectrometers and we summarise how these can be coordinated. This 3D spectroscopy is a new research domain that will aid the understanding of the complex flows that take place throughout the solar atmosphere.
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The Solar coronal heating problem remains a persistent challenge in astrophysics. Parker postulated back in 1988 that the heating of corona should be dominated by small energy dissipation events, referred to as nanoflares. However, there have not yet been any confirmed observations of individual nanoflares. Hi-C reported unique bright points with energies ranging between log[E(ergs)] = 24-25. Those brightenings were also identified in AIA passbands. Here, I will describe 0-D hydrodynamical simulations to forward model these tiny brightenings, study their energetics, and discuss possible implications for coronal heating.
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GREGOR is Europe’s largest solar telescope. It has undergone significant upgrades and changes from 2018-2020 to improve the image quality, instrumentation, and operation. Particularly, a complete redesign of the optics laboratory was performed by KIS to obtain diffraction-limited images from the blue to the infrared. The new optics setup was completed while “trapped” on the mountain during the lockdown and we obtained first light images in July 2020. In this talk, I will summarize the most important updates, explain the new optics setup, and show the improved images.
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The details study of the solar activity and variability on long term data series is an important element on the understanding of the solar dynamics and evolution. In addition, their activity influences several aspects of our lives, such as climate, communications, energy, aviation, and many other fields, sustaining and threatening, simultaneously, our entire technologically-based way of life. Hence, it is paramount to secure the continuity of unbroken and self-consistent data series of solar observations and its study. The associated physical processes and structures on the Sun span over a wide range of values regarding their lifetimes, intensities, and spatial scales. Ideally, to study all these different structures in detail, we need facilities that allow us to observe the full solar disk and spectrum with very high spectral, spatial, and temporal resolutions. Unfortunately, due to technical limitations that cannot be done and there are clear trade-offs to deal with. Even today, elements like spatial resolution versus field of view (FoV), spectral coverage versus temporal resolution, observation of spectral lines in local or non-local thermodynamic equilibrium (depending on the science driver) are among the obstacles that scientists need to keep in mind as limitations for their work. State-of-the-art solar telescopes like the US American 4-meter telescope DKIST, the German GREGOR telescope, or the Solar Orbiter, plus the next generation facilities like the balloon borne SUNRISE III mission, or the future European Solar Telescope will observe the Sun with unprecedented spatial and spectral resolutions. Even though they will use the most advanced technology they do not cover all possible modes of observation. Some key aspects like small FoVs, in some cases limited number of spectral lines or the short lifetime of instruments are among those that some other types of instruments can cover. The spectroheliograph of OGAUC is one of the most durable solar instruments still operating. Having been upgraded only twice, one for new optics and another for digital image recording, it keeps daily observations since 1927. Until now this instrument has been used to study structures visible in the solar atmosphere from their intensity images at specific wavelengths, ignoring most of the visible spectral range. One of the main reasons for that is the lack of tools to extract more information from that type of observations that need to be analysed in Non-Local Thermodynamic Equilibrium (NLTE) regime. However, with the new generation of spectropolarimetric inversion codes, we are now able to invert NLTE spectral lines to extract information about the temperature, the velocity and the magnetic field vector. Such analysis is already possible with several of the aforementioned telescopes, but they do not cover all the possible observing modes. In this project, we propose to take advantage of the operating infrastructure of the OGAUC spectroheliograph and to upgrade it, improving its spectral resolution, adding other spectral regions of interest and increasing the spatial sampling and add polarimetric sensitivity. Due to its flexibility, long term run and set of observed spectral lines and polarimetric sensitivity it will be a competitive state of the art instrument competing with the other solar synoptic (full disk) ground-based spectroheliograph of its category in the. This upgrade, in combination with the new NLTE inversion codes and neural network techniques will allow us to probe at chromospheric and photospheric heights the solar temperature, velocity and magnetic field.
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The chromospheric hydrogen Lyman-alpha line at 1216A is the brightest emission line in the solar spectrum, and yet studies of solar flares at this wavelength have been scarce in the literature over the past 50 years. The study of Lyman-alpha is important for understanding space weather effects as changes in the Sun’s Lyman-alpha output can drive changes in the dynamics and composition of planetary atmospheres. Lyman-alpha is also a significant radiator of solar flare energy, providing an important diagnostic of energy release and transport processes. Milligan et al. (2020) published a statistical study of ~500 M- and X-class flares using GOES/EUVS data, showing that although the Lyman-alpha irradiance increases by only a few percent during large events, it can radiate up to 100 times more energy than the corresponding X-rays. Flares that occurred closer to the solar limb, however, were found to exhibit a smaller Lyman-alpha enhancement relative to those on the disk due to opacity and/or foreshortening effects. It was also shown that acoustic oscillations in the chromosphere can be detected through Lyman-alpha flare observations, and that impulsive Lyman-alpha emission, not X-rays, can induce currents in the E-layer of Earth’s ionosphere. A follow-up study now includes B- and C-class flares, which although not readily observable in disk-integrated measurements, can be investigated using a superposed epoch analysis. Despite increases of <1% above the solar background, a clear centre-to-limb variation was found in agreement with larger events. These findings should serve as a baseline for the advent of new Lyman-alpha flare observations and advanced numerical simulations that will become available during Solar Cycle 25.
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The Hα spectral line is a well-studied absorption line revealing properties of the highly structured and dynamic solar chromosphere. The presented work is based on high-spectral resolution Hα spectra obtained with the echelle spectrograph of the Vacuum Tower Telescope (VTT) located at Observatorio del Teide (ODT), Tenerife, Spain. The number of spectra accumulated at VTT over one observing day easily reaches up to millions. Hence, we require tools to identify and classify spectra with minimal human intervention. I will present exploratory work, which provides the framework and some ideas on how to tailor a classification scheme towards specific spectral data and science questions. t-distributed Stochastic Neighbor Embedding (t-SNE) is a machine learning algorithm, which is used for nonlinear dimensionality reduction. In this application, it projects Hα spectra onto a two-dimensional map, where it becomes possible to classify the spectra according to results of Cloud Model (CM) inversions. The CM parameters optical depth, Doppler width, line-of-sight velocity, and source function describe properties of the cloud material. Initial results of t-SNE indicate its strong discriminatory power to separate quietSun and plage profiles from those that are suitable for CM inversions. Furthermore, I will discuss our choice of various t-SNE parameters, the impact of seeing on classification, the results arising from various types of input data, and the link of the identified clusters to chromospheric features. Although t-SNE proves to be efficient in clustering high-dimensional data, human inference is required at each step to interpret the results.
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Solar spicules are one of the dominant dynamic phenomena of the lower solar atmosphere. Here, we show our results on modelling the propagation of such localised jets driven by a momentum pulse as the exciting force near photospheric heights. Using the MPI-AMRVAC code to perform 2D MHD simulations in an idealised stratified solar atmosphere, we investigate how key parameters (e.g., driver time, equilibrium magnetic field strength, velocity amplitude of driver and tilt with respect to the magnetic field) determine the morphology of these small-scale solar jets. A parametric study is carried out and using jet tracking software we analyse the jet properties (e.g., widths, apex heights, etc). We find that jet boundary deformation occurs naturally due to speeds involved in driving these jets within the range of spicule heights that could be then a possible alternative explanation for the appearance of transverse motions (both axisymmetric and non-axisymmetric deformations). By resolving structures up to 10 km, we also find unforeseen substructures inside the spicular jet beam. We propose observers to confirm this latter finding that may be challenging due to current spatial resolution limits.
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Using multi-wavelength imaging observation obtained from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO), we study the evolution of Kelvin-Helmholtz (K-H) instability in a fan-spine magnetic topology. This fan-spine configuration is situated near the Active Region 12297 and is rooted in a nearby sunspot. The two layers of the cool plasma flows lift up from the fan plane in parallel and interact with each other at the maximum height of the elongated spine in the lower corona. The first layer of the plasma flow (F1) moving with a slow velocity (5 km/s) reflected from the spine’s field lines. Subsequently second layer of plasma flow (F2) with impulsive velocity (114-144 km/s) interacts with the first layer at the maximum height and generating the shear motion , which is responsible for the evolution of the Kelvin-Helmholtz instability inside the elongated spine. The amplitude and characteristics wavelength of the K-H unstable vortices increases, which satisfy the linear growing mode of this instability. Using linear stability theory of the K-H instability, we calculate the Alfvén velocity in the lower layer. We conjecture that the estimated shearing velocity is higher than the estimated the Alfvén velocity in the second denser layer, which also satisfies the classical criterion of K-H instability. The fan-spine configuration possesses magnetic field and sheared velocity component, we estimate the parametric constant [Λ≥1] which confirms that the velocity shear dominates and the linear phase of the K-H instability is evolved. The present observation indicate that in the presence of complex magnetic field structuring and plasma flows, the K-H instability evolve in the fan-spine configuration may evolve the rapid heating, and connectivity changes may occur due to the fragmentation via the K-H instability. It also act as a rapid mechanism to transfer the mass and energy release between two distinct regions separated by the fan-spine configuration.
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The chromosphere of the umbra of sunspots is a remarkably dynamic layer featuring extremely fine sub-arcsec structure. Such structures appear dark against enveloping umbral flashes, but also bright before or after a flash, other features still are bright throughout. Only recently did we start understanding such fine features and semi-empirical modelling is converging with simulations to provide insight, not only into such fine structure, but also into the umbral flash phenomenon itself. The observational evidence weighs overwhelmingly towards a strong corrugation of the umbra where the material in short dynamic fibrils over-extends in a column of upflowing material while the adjacent areas flash. The delayed small-scale umbral brightenings at the bottom of such columns are an out-of-phase flash where the late-stage downflowing column meets the upflowing under-layers. Recent inversions using NICOLE at both umbral flashes and small-scale brightenings result in a downflow over upflow stratification perfectly bridging the transition of downflowing fibrils to upflowing fibrils as well as red-shifted absorption cores to blue-shifted absorption cores in the broader surroundings. Locally, each inverted column is remarkably similar in velocity profile to those from forward modelling, provided the formation height of the observed Ca II 8542 line is slightly lower in the Sun than in the simulations. Conspicuously, resonant cavities naturally cause the upper downflowing layers to become visible in forward modelling and the top-layer downflows to last longer than otherwise. Open questions, and how these can be addressed by future observations, are briefly discussed.
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We present the state-of-art detection method of three-dimensional vortices and apply it to realistic magneto-convections simulations performed by the MURaM code. The detected vortices extend from the photosphere to the low chromosphere, presenting similar behaviour at all height levels. The vortices concentrate the magnetic field, and thereby the plasma dynamics inside the vortex is considerably influenced by the Lorentz force. Rotational motions also perturb the magnetic field lines, but they lead to only slightly bent flux tubes as the magnetic field tension is too high for the vortex flow to significantly twist the magnetic lines. We find that twisted magnetic flux tubes are created by shear motions in regions where plasma-beta>1, regardless of the existence of flow vortices.
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We study the dynamics of plasma along the legs of an arch filament system (AFS) observed with relatively high-cadence spectropolarimetric data from the ground-based solar GREGOR telescope (Tenerife) using the GREGOR Infrared Spectrograph in the He I 10830 Å range. The temporal evolution of the plasma of an AFS was followed using the chromospheric He I 10830 Å triplet and Si I 10827 Å. Measurements of vector magnetic fields in the solar chromosphere, especially in AFS, are extremely scarce, but very important. The magnetic field configuration reveals how AFSs are sustained in the chromosphere and hints at their formation, evolution, and disappearance. The magnetic field in the AFS follows loop-like structures traced by chromospheric absorption lines. However, if magnetic field lines follow chromospheric threads as seen by filtergrams of H⍺, Ca II, or He I, is still not fully resolved. Previous studies have modeled AFS as multiple flux ropes with mixed signs of helicity consistently with the observed multiple filament bundles constituting AFS. Nevertheless, further spectropolarimetric observations are needed to address this issue. Many spectral lines are sensitive to the atmospheric parameters up to the upper chromosphere. Moreover, when combined with photospheric Zeeman sensitive spectral lines, one can infer the topology of the magnetic field from the bottom of the solar atmosphere to the chromosphere. In this talk, we are going to follow the nature of AFSs by reconstructing the magnetic field configuration of an EFR from the very beginning and follow its evolution and dynamics to support current AFS models. To that aim we used the spectropolarimetric data available at the upper photosphere (Si I) and the upper chromosphere.
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We report 2D MHD simulations of the large-amplitude oscillations (LAOs) in the solar prominences performed with MHD code Mancha. We aim to study the properties of LAOs using high-resolution simulations in a simple 2D magnetic configuration that contains a dipped part. We loaded the dense prominence plasma in the dips region. In order to excite oscillations, we used a perturbation directed along the magnetic field. For the same numerical model, the four spatial resolutions were considered: 240, 120, 60, and 30 km. The longitudinal LAOs (LALOs) are strongly damped even in the high-resolution simulation in the region of the weaker and more curved magnetic field (at the center and bottom of the prominence). At the prominence top, the oscillations have relatively longer damping times. Furthermore, during the first 100 minutes, the longitudinal velocity shows growing with respect to its initial amplitude. The amplification becomes even more significant in the experiments with high-resolution. The damping and amplification mechanisms involved in our experiments can be important for explaining the observed amplification and attenuation of the LALOs.
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Penumbral filaments do not form naturally in MHD simulations of sunspots. This is typically circumvented by modifying the top boundary: the field is made 2-3 times more horizontal than a potential field configuration. In this talk, I will explore the possibility that penumbral filament formation is governed by the subsurface structure of sunspots. We conducted a series of numerical experiments where we used flux tubes with different initial curvatures to study the effect of the fluting instability on the subsurface structure of spots using the MURaM code. We find that the curvature of a flux tube indeed determines the degree of fluting the flux tube will undergo—the more curved a flux tube is, the more fluted it becomes. In addition, sunspots with strong curvature have strong horizontal fields at the surface and therefore readily form penumbral filaments. The fluted sunspots eventually break up from below, with lightbridges appearing at the surface several hours after fluting commences. We also propose that intruding lightbridges can be used as tracers of the subsurface magnetic field.
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In this seminar, we aim to show why Fast/Alfvén waves couple at the solar surface. We will also show that the polarisation of the waves changes upon reflection at the solar surface. Finally, we will test the validity of line-tied boundary conditions for highly phase-mixed Alfvén waves. For most parameters, line-tied boundary conditions provide a good approximation. However, for highly phase-mixed waves, the coronal transverse length scales can be shorter than the corresponding parallel length scales in the chromosphere. In that case, we find that the line-tied model produces unphysically large boundary layers. Hence, we have the counter-intuitive result that the length scales parallel to the solar surface play a key role in determining the validity of line-tied boundary conditions.
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In this seminar, we aim to present the results of two recent works centred at the use of spectropolarimetric data obtained with the CRISP instrument at the SST in the Ca II 845.2 nm line. With these observations, we obtain information about the magnetic field present in chromospheric spicules and coronal rain clumps. For this purpose, we have used the Weak-field approximation (WFA), which albeit being computationally simple to implement, needs careful assessment of the conditions of the plasma to be correctly applied. Magnetic fields of the order of hundreds of Gauss are inferred. We also combine the Ca II 845.2 nm observations with simultaneous Hα observations to estimate the temperature and non-thermal velocity of the plasma in coronal rain and spicules using the observed Doppler amplitude.
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Small, highly frequent flares are thought to contribute to heating the Sun’s atmosphere, particularly in active regions. This impulsive energy release would heat plasma >5 MK and accelerate electrons, producing weak thermal and non-thermal signatures that could be observed by a very sensitive X-ray telescope. No such solar telescope exists currently so we use the Nuclear Spectroscopic Telescope Array (NuSTAR), an astrophysical X-ray telescope, with focusing optics imaging spectroscopy providing a unique sensitivity for observing the Sun above 2.5 keV. In this seminar, I will present an overview of the discoveries from NuSTAR solar observations where decreasing solar activity between cycle 24 and 25 has allowed GOES sub-A class microflares to be observed regularly within, and small brightenings outside, active regions. In particular, I will describe several X-ray microflares from a recently emerged active region, AR12721, that were observed on 2018 September 9-10 with NuSTAR. In combination with SDO/AIA, I will discuss the temporal, spatial, and spectral evolution of these sub-A class microflares and show that temperatures up to 10 MK are reached. Using SDO/HMI, I also present evidence of photospheric magnetic flux cancellation/emergence at the footpoints in 8 NuSTAR microflares.
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The Mg I b2 line at 5173 Å forms over a large range of heights but its core, which forms under conditions of non-local thermodynamic equilibrium, is most sensitive to heights near the temperature minimum, a region of the solar atmosphere that has not been sufficiently explored. The next-generation solar observatories will have access to this spectral line and will allow for multi-line observations to study the different layers of the solar atmosphere simultaneously and with unprecedented polarimetric sensitivity. We will present a morphological classification of the intensity and circular polarization profiles of this spectral line at high-spatial-resolution, using observations from the Swedish 1-m Solar Telescope. We will also discuss the results of the weak field approximation applied to the Mg I b2 line, and their comparison with inversion results of the Fe I 6173 Å line to understand how the magnetic field changes with height in the solar atmosphere.
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Element abundance signatures have long been used as tracers of physical processes throughout astrophysics. Understanding the spatial and temporal variations in the composition of the solar corona provides insight into how matter and energy flow from the solar chromosphere out into the heliosphere as well as from the chromospheres of solar-type stars into their astrospheres. In this work, we investigate the spatial distribution of highly varying plasma composition around one of the largest sunspots of solar cycle 24. Observations of the photosphere, chromosphere, and corona are brought together with magnetic field modeling of the sunspot in order to probe the conditions that regulate the degree of plasma fractionation within loop populations of differing connectivities. We find that, in the coronalmagnetic field above the sunspot, variation in plasma composition is highly structured, with extremes in the level of fractionation among the distinct loop populations. Loops above the umbra contain unfractionated plasma, i.e. photospheric composition, while coronal loops rooted in the penumbra contain fractionated plasma, with the highest levels observed in the loops that connect within the active region. The distribution of the highly fractionated plasma appears to be correlated with the spatial locations at which intrinsic magnetic perturbations are identified in high spatial resolution spectropolarimetric observations of the solar chromosphere. Tracing field lines from regions of highly fractionated plasma in the corona to locations of magnetic perturbations detected in the chromosphere shows that they are magnetically linked. These results indicate a direct connection between sunspot chromospheric activity and observable changes in coronal plasma composition. We interpret our findings in the wider context of coronal heating and the ponderomotive force model of elemental fractionation.
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We present high-resolution and multiline spectropolarimetric observations of a C2-class solar flare (SOL2019-05-06T08:47). The rise, peak, and decay phases of the flare were recorded continuously and simultaneously in the Ca II K, Ca II 8542 Å, and Fe I 6173 Å lines with the CRISP and CHROMIS instruments at the Swedish Solar Telescope (SST). At the flare footpoints, a non-LTE multiline inversion code (STiC) was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity, and microturbulent velocity. The temporal analysis of the inferred temperature at the flare footpoints shows that the flaring atmosphere from log τ500 ∼ −2.5 to −3.5 is heated up to 7 kK, whereas from log τ500 ∼ −3.5 to −5 the inferred temperature ranges between ∼7.5 kK and ∼11 kK. During the flare peak time, the LOS velocity shows both upflows (evaporation) and downflows (condensation) around the flare footpoints in the upper chromosphere and lower chromosphere, respectively. Moreover, the temporal analysis of the LOS magnetic field exhibits a maximum change of ∼600 G. After the flare, the LOS magnetic field decreases to the non-flaring value, exhibiting no permanent or step-wise change. The analysis of response functions to the temperature, LOS magnetic field, and velocity shows that the Ca II lines exhibit enhanced sensitivity to the deeper layers compared to the non-flaring atmosphere. We also estimated the radiative losses from the singly ionized Ca and Mg atoms using the semi-empirical model atmosphere inferred from the inversion of the CRISP and CHROMIS dataset. The obtained radiative losses range from 50 to 180 kW/m^2 near the flare footpoints. Our observations illustrate that even a less intense C-class flare can heat the deeper layers of the solar chromosphere, mainly at the flare footpoints, without affecting the photosphere.
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The Space Weather effects in the near-Earth environment as well as in atmospheres of other terrestrial planets arise by corpuscular radiation from the Sun, known as the solar wind. The solar magnetic fields govern the solar corona structure. Magnetic-field strength values in the solar wind sources - key information for modeling and forecasting the Space Weather climate - are derived from various solar space- and ground-based observations, but, so far not accounting for specific types of radio bursts. These are “fractured” type II radio bursts attributed to collisions of shock waves with coronal structures emitting the solar wind. Here, we report about radio observations of two “fractured” type II bursts to demonstrate a novel tool for probing of magnetic field variations in the solar wind sources. These results have direct impact on interpretations of this class of bursts and contribute to the current studies of the solar wind emitters.
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Solar flares originate from active regions (ARs) hosting complex and strong bipolar magnetic fluxes. Forecasting the probability of an AR to flare and defining reliable precursors of intense flares, i.e., X- or M-class flares, are extremely challenging tasks in the space weather field. In this talk, we focus on two metrics as flare precursors, the unsigned flux R*, tested on MDI/SOHO data and calibrated for higher spatial resolution SDO/HMI maps, and a novel topological parameter D representing the complexity of a solar active region. The parameter D is based on the automatic recognition of magnetic polarity inversion lines (PILs) in identified SDO/HMI ARs and is able to evaluate their magnetic topological complexity. We use both a heuristic approach and a supervised machine-learning method to validate the effectiveness of these metrics to predict the occurrence of X- or M-class flares in a given solar AR during the following 24 hr period. Our feature ranking analysis shows that both parameters play a significant role in prediction performances. Moreover, the analysis demonstrates that the new topological parameter D is the only one, among 173 overall predictors, that is systematically ranked within the top 10 positions. Reference: Cicogna et al., Flare-forecasting Algorithms Based on High-gradient Polarity Inversion Lines in Active Regions, ApJ, 915, id.38, 2021
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Magnetic reconnection is widely accepted to be a major contributor to nonthermal particle acceleration in the solar atmosphere. We investigate particle acceleration in two evolving field geometries: first in an isolated tearing current sheet, then in a full-scale coronal jet. Both geometries involve 3D reconnection with at least one magnetic null point. A test-particle approach is employed, using electromagnetic fields from magnetohydrodynamic (MHD) simulations of these geometries. Using this method, we examine the trajectories of high-energy protons and electrons injected near reconnecting null points and how the directionality of their acceleration differs. We will discuss what the ejection and impact patterns of heliosphere and photosphere-incident particles respectively can tell us about the location, size and shape of field structures that are formed in tearing current sheets during null-point reconnection in the solar corona. We will also consider how we may observe the simulated differences between proton and electron impact patterns.
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Invoking the effects of thermal conductivity, compressive, viscosity, radiative losses, and heating-cooling misbalance, we derive the new general dispersion relation for the propagating slow MHD waves in the solar corona and solve it to determine the phase shifts of density and temperature perturbations along with their dependence on the equilibrium parameters of the plasma such as the background density and temperature. We also derive a new generalised mathematical expression for the polytropic index using the linear MHD model and find that in the presence of thermal conduction alone it remains close to its classical value for all the considered equilibrium density and temperature observed in typical coronal loops. Under the considered heating and cooling models, we find that the expected polytropic index can be matched with the observed value of 1.1 ± 0.02 in typical coronal loops if the thermal conductivity is enhanced by an order of magnitude compared to its classical value. We also explore the role of different heating functions for typical coronal parameters and find that although the polytropic indices remain close to 5/3, the phase difference between density and temperature perturbations is highly dependent on the form of heating function.
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Abstract: The solar coronal heating in quiet Sun (QS) and coronal holes (CH), including solar wind formation, are intimately tied by magnetic field dynamics. Thus, a detailed comparative study of these regions is needed to understand the underlying physical processes. CHs are known to have subdued intensity and larger blueshifts in the corona. This work investigates the similarities and differences between CHs and QS in the chromosphere using the Mg II h & k, C II lines, and transition region using Si IV line, for regions with identical absolute magnetic flux density (|B|). We find CHs to have subdued intensity in all the lines, with the difference increasing with line formation height and |B|. The chromospheric lines show excess upflows and downflows in CH, while Si IV shows excess upflows (downflows) in CHs (QS), where the flows increase with |B|. We further demonstrate that the upflows (downflows) in Si IV are correlated with both upflows and downflows (only downflows) in the chromospheric lines. CHs (QS) show larger Si IV upflows (downflows) for similar flows in the chromosphere, suggesting a common origin to these flows. These observations may be explained due to impulsive heating via interchange (closed-loop) reconnection in CHs (QS), resulting in bidirectional flows at different heights, due to differences in magnetic field topologies. Finally, the kinked field lines from interchange reconnection may be carried away as magnetic field rotations and observed as switchbacks. Thus, our results suggest a unified picture of solar wind emergence, coronal heating, and near-Sun switchback formation. References: — “On the formation of solar wind & switchbacks, and quiet Sun heating”: Vishal Upendran and Durgesh Tripathi 2021, ApJ (accepted) — “Properties of the C II 1334 Å Line in Coronal Hole and Quiet Sun as Observed by IRIS”: Vishal Upendran and Durgesh Tripathi 2021, ApJ, 922, 112
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Internal gravity waves (IGWs) are buoyancy-driven waves common in the Earth’s atmosphere and oceans. IGWs have also been observed in the Sun’s atmosphere and are thought to play an important role in the overall dynamics of the solar atmosphere. They supply bulk of the wave energy for the lower solar atmosphere, but their existence and role in the energy balance of the upper layer remains unclear. Using radiation-magnetohydrodynamic (R-MHD) simulations, we study naturally excited IGWs in realistic models of the solar atmosphere. In this talk, we discuss some of our recent results on the influence of the Sun’s magnetic field on the propagation of IGWs and their energy transport. Our analysis suggests that the IGWs are generated independent of the mean magnetic property of the atmosphere. However, their propagation into higher layers is strongly affected by the presence and the topology of the magnetic field. We discuss how IGWs may play a significant role in the heating of the chromospheric layers in regions where horizontal fields are thought to be prevalent, like the internetwork region.
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Observational precursors of large solar flares provide a basis for future operational systems for forecasting. We studied the evolution of the normalized emergence (EM), shearing (SH), and total (T) magnetic helicity flux components for 14 flaring (with at least one X-class flare) and 14 nonflaring (
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Magnetohydrodynamic (MHD) waves are thought to be one of the key mechanisms for transferring energy and momentum through the Sun’s atmosphere, hence maintaining the temperature profile of the outer atmospheric layers. Here, we have studied small-scale chromospheric bright features, exhibiting oscillations in brightness temperature, size, and horizontal velocity, in Bands 3 (∼3 mm) and 6 (∼1.2 mm) of 2 seconds cadence solar observations with ALMA, as well as in associated synthetic lines from a Bifrost simulation, degraded to match the ALMA’s spatial and temporal resolutions. In total, 486 and 235 features were analysed in the observations and simulations, respectively. Periods of the oscillations and phase angles between the perturbations in any of the two parameters were characterised by means of wavelet analysis. As a result, median periods were obtained for the oscillations on the order of 90 s (Band 3) and 64 s (Band 6) for brightness temperature, 82 s (Band 3) and 56 s (Band 6) for size and, 65 s (Band 3) and 52 s (Band 6) for horizontal velocity. Phase relations between the high-frequency oscillations in brightness temperature and size suggest the presence of fast and slow MHD sausage modes in the small magnetic structures. In addition, the high-frequency fluctuations in transverse displacement are likely Alfvénic and can be representative of MHD kink mode. Furthermore, we have compared the outcomes between the two ALMA frequency bands as they are considered to be formed at distinct heights in the solar chromosphere and have used the simulations to discuss the context of the observational results. Finally, this study confirms the diagnostic potential of solar ALMA observations with very good cadence and resolution, as well as their essential role as complementary with respect to other diagnostics.
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The 3D MHD global modeling is a powerful tool to test all the possible candidate physical processes responsible for the formation and evolution of the corona and heliosphere. To fully understand the possible role of each of these mechanisms, we need a validation process where the output from the simulations is quantitatively compared to the observational data. In this work, we present the results from our validation process applied to the wave turbulence driven 3D MHD corona-wind model WindPredict-AW. At this stage of the model development, we focus the work to the coronal regime in quiescent condition. We analyze three simulations results, which differ by the boundary values. We use the 3D distributions of density and temperature, output from the simulations at the time of around the first Parker Solar Probe perihelion (during minimum of the solar activity), to synthesize both extreme ultraviolet (EUV) and white light polarized (WL pB) images to reproduce the observed solar corona. For these tests, we selected AIA 193 A, 211 A and 171 A EUV emissions, MLSO K-Cor and LASCO C2 pB images obtained the 6 and 7 November 2018. We then make quantitative comparisons of the disk and off limb corona. We show that our model is able to produce synthetic images comparable to those of the observed corona.
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Solar pores have long been documented as efficient magnetic conduits for propagating magnetohydrodynamic wave energy from the photosphere into the outer regions of the solar atmosphere. Observations of pores often contain isolated and/or unconnected structures, which prevents the statistical examination of wave activity inter- play as a function of atmospheric height. Here, using high resolution observations acquired by the Dunn Solar Telescope, we examine photospheric and chromospheric wave signatures stemming from a unique collection of magnetic pores originating from the same decaying sunspot. A common driver for waves in each pore is detected, allowing for an unprecedented study of each wave guide using novel methods, to gain insight into their underlying attributes, with a view to next-gen solar observatories.
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Several generalizations of the well-known fluid model of Braginskii (Rev. of Plasma Phys., 1965) are considered. We use the Landau collisional operator and the moment method of Grad. We focus on the 21-moment model that is analogous to the Braginskii model, and we also consider a 22-moment model. Both models are formulated for general multi-species plasmas with arbitrary masses and temperatures, where all the fluid moments are described by their evolution equations. The 21-moment model contains two “heat flux vectors” (3rd and 5th-order moments) and two “viscosity-tensors” (2nd and 4th-order moments). The Braginskii model is then obtained as a particular case of a one ion-electron plasma with similar temperatures, with de-coupled heat fluxes and viscosity-tensors expressed in a quasi-static approximation. We provide all the numerical values of the Braginskii model in a fully analytic form (together with the 4th and 5th-order moments). For multi-species plasmas, the model makes calculation of transport coefficients straightforward. Formulation in fluid moments (instead of Hermite moments) is also suitable for implementation into existing numerical codes. It is emphasized that it is the quasi-static approximation which makes some Braginskii coefficients divergent in a weakly-collisional regime. Importantly, we show that the heat fluxes and viscosity-tensors are coupled even linearly and that the fully contracted (scalar) perturbations of the 4th-order moment, which are accounted for in the 22-moment model, modify the energy exchange rates.
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Relaxation of braided coronal magnetic fields through reconnection is thought to be a source of energy to heat plasma in active region coronal loops. However, observations of active region coronal heating associated with untangling of magnetic braids remain sparse. One reason for this paucity could be the lack of coronal observations with sufficiently high spatial and temporal resolution to capture this process in action. Using new high spatial resolution (250–270 km on the Sun) and high cadence (3–10 s) observations from the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter we observed untangling of small-scale coronal braids in different active regions. The untangling is associated with impulsive heating of the gas in these braided loops. We assess that coronal magnetic braids overlying cooler chromospheric filamentary structures are perhaps more common. Furthermore, our observations show signatures of both spatially coherent and intermittent coronal heating during relaxation of magnetic braids. Our study reveals the operation of both more gentle and explosive modes of magnetic reconnection in the solar corona. In this talk, we present these new EUI observations and discuss the implications for magnetic braiding associated coronal heating.
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The analytical and numerical modelling of the behaviour of magnetohydrodynamic (MHD) waves in various magnetic geometries is a constantly evolving, active area of research within the field of solar magneto-seismology. This presentation focuses on MHD wave propagation and instabilities in a family of asymmetric Cartesian waveguide models. Thanks to the introduction of various sources of asymmetry (background density, magnetic field or flow speed), this generalisation of classical (symmetric) slab geometries allows us to refine our models of several features in the richly structured solar atmosphere. Including background asymmetry in these configurations influences the phase speeds and cut-off frequencies of the eigenmodes, and, in the case of flow asymmetry, it can also change the threshold for the onset of the Kelvin-Helmholtz instability. The asymmetric nature of the models also allows us to develop solar magneto-seismologic tools and provide efficient methods for obtaining further information about the solar plasma (e.g. magnetic bright points).
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The rapid increase of the horizontal magnetic field (Bh) around the flaring polarity inversion line is the most prominent photospheric field change during flares. It is considered to be caused by the contraction of flare loops, the details behind which is still not fully understood. Here we investigate the Bh increase in 35 major flares using HMI high-cadence vector magnetograms. We find that the Bh increase is always accompanied by the increase of field inclination. It usually initiates near the flare ribbons, showing a step-like change in between the ribbons. In particular, its evolution in the early flare phase shows a close spatiotemporal correlation to flare ribbons. We further find that the Bh increase tends to have similar intensity in confined and eruptive flares but a larger spatial extent in eruptive flares in a statistical sense. Its intensity and timescale have inverse and positive correlations to the initial ribbon separations, respectively. The results altogether are well consistent with a recent proposed scenario that suggests that the reconnection-driven contraction of flare loops enhances the photospheric Bh according to the ideal induction equation, providing statistical evidence of the reconnection-driven origin for the Bh increase for the first time.
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Spicular jets are highly elongated chromospheric plasma features that are believed to transport momentum to the solar wind and non-thermal energy to heat the atmosphere. At any given time, it is estimated that about 3 million spicules are present on the Sun. We find an intriguing parallel between the simulated spicular forest in a solar-like atmosphere and the numerous jets of polymeric fluid in the laboratory when both are subjected to harmonic forcing. In our radiative (both 2D and 3D) MHD simulations with sub-surface convection, the solar surface oscillations are excited similarly to those harmonic vibrations. A forest of spicules are formed in our simulations with heights ranging between 6 and 25 Mm, bearing substantially closer resemblance to clusters of jets observed in the solar atmosphere. Taken together, the numerical simulations of the Sun and the laboratory fluid dynamics experiments provide insights into the mechanism underlying the ubiquity of jets. The insight provided by the polymeric fluid experiments when combined with the commonalities with the numerical solar MHD simulations is that four basic ingredients are sufficient to assemble a forest of spicules on the Sun by non-linear wave development.
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Solar flares are intense eruptions of radiation from the solar surface and they are closely associated with Coronal Mass Ejections (CMEs) that are known to inflict radio and magnetic disturbances on the Earth. Recent studies on the forecasting of solar flares involving the 3D extrapolation of the pre-flare magnetic configuration have identified a particular height range between the photosphere and the lower solar corona where the flare onset prediction time can be maximised. This novel concept of an ‘optimal height range’ has been developed by studying the variation of predictors as a function of height above the photosphere and time before the eruption of a flare. Recent studies have even established that the weighted horizontal gradient of the line of sight component of the magnetic field, i.e. the ‘WGM morphological parameter’ and ‘Magnetic Helicity’ are good predictors. We have applied the concept of optimal height to a set of four predictors related to Magnetic Polarity Inversion Lines (MPILs); namely (i) the maximum length of MPILs, (ii) the total length of MPILs, (iii) the total absolute magnetic flux and (iv) R-value. We discuss the need for such predictors and our preliminary observations by assessing their behaviour as a function of height (above the photosphere) and time (before the flare) for a select few Active Regions.
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Low-frequency radio observation provides a unique viewing point for solar and spaceweather studies, including the inspection of solar energetic particles and the diagnostics for background plasmas. Solar radio bursts are generated from high energy particles pacing through the solar atmosphere and inner heliosphere interacting with the background plasma. The Low-frequency array (LOFAR) is a radio telescope composed of massive numbers of antennas spreading all over Europe, it is currently the largest radio telescope in the frequency range of 10-240MHz. In this talk, I present the recent proceedings in solar and spaceweather radio observations with LOFAR's high resolution and sensitivity, including the imaging and spectroscopy of solar radio bursts, using bright quasar to perform sounding detection for the inner-heliosphere, and imaging of the quiet Sun.
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The magnetic field of solar-type stars is generated and sustained through an internal dynamo process. This process is mostly determined by the combined action of turbulent convective motions and differential rotation. It can sometimes lead to magnetic cyclic variabilities, like the 11-years solar cycle. Evidence of magnetic cycles have been detected for other solar-type stars as well, ranging from a few years to a few tens of years. How are these cycles controlled? Observations and stellar evolution models show that solar-like stars spin-down during their main-sequence. In parallel, numerical simulations of these stars show that different regimes of differential rotation can be reach and are characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby numbers (slow rotators), which grows when the rotation spins-down. If this regime appears during the main sequence, we may wonder how the dynamo process will be impacted, especially if our Sun is in such a transition. In particular, can slowly rotating stars have magnetic cycles? We performed a numerical multi-D parametric study with the STELEM and ASH codes to understand the magnetic field generation of solar-type stars under various differential rotation regimes. We particularly focused on the energy transfers powering these stellar dynamos, and on the existence of magnetic cycles for different stages of the main sequence. We find that short cycles are favoured for small Rossby numbers (fast rotators), and long cycles for intermediate (solar-like) Rossby numbers. We further assess that energy transfers can reach up to 3% of the stellar luminosity to sustains these dynamos, and ultimately powering surface eruptive events. Finally, we find that anti-solar rotating stars (high Rossby numbers) can only sustain magnetic cycles for specific dynamo processes. This led us to develop a theoretical criterion to select anti-solar candidates with the perspective to bring new constraints on our models, stellar evolution, and more particularly on the future of the Sun.
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Waves play an important role in the heating of the upper atmosphere of the Sun. Different features observed over sunspots at different atmospheric heights host a variety of waves, such as the 5-minutes photospheric oscillations, the 3-minute chromospheric oscillations, umbral flashes and waves, running penumbral waves (RPWs), and propagating coronal waves. Although these oscillations and waves have been studied for decades, we are still far from understanding the physics behind their origin and the possible coupling among them. In this talk, the speaker will talk on multiwavelength imaging observations that will provide pieces of evidence on the nature and extent of the coupling among different sunspot waves observed in the different layers of the solar atmosphere.
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Some observations show that jets impact solar filaments producing large-amplitude oscillations in these structures. The seminar will consist of two parts. In the first part the model proposed by Luna & Moreno-Insertis (2021) will be presented. This work suggests a scenario for the generation of the jet and its interaction with the filament. It was found that there are two phases in the jet, eruptive and quiescent, associated with two different reconnection phases. Jet flows emanating from the reconnection regions collide with the prominence producing significant displacements of its heavy mass and large-amplitude oscillations are established. In the second part of the seminar, a recent work will be shown in which two observational case studies are analysed. The jet environment and how it impacts the filament is studied in detail. The oscillations of the filament are also studied and seismology is applied to infer the structure and intensity of the magnetic field. We conclude that both cases presented are consistent with the numerical model.
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The Dark Halos (DHs) are regions of reduced emission in the neighborhood of solar active regions (ARs). Since their discovery (Hale & Ellerman, 1903), DHs have been called with various names, the first one being circumfacules (Deslanders, 1930), as they were first identified in the chromosphere as faint areas surrounding plages in the Ca II K-line, and later also in the Ca II 8542 Å line (D’Azumbuja, 1930). Bumba & Howard (1965) proposed that the Ca II circumfacules are composed of dark features corresponding to dark Hα fibrils, and this suggestion was supported by different authors (e.g. Rutten et al. 2007, Pietarila et al. 2009). A typical chromospheric fibrillar DH normally has a counterpart also in the other layers of the solar atmosphere. Indeed, with the advent of UV and EUV observations, it has been possible to observe dark regions around ARs also in a wide range of spectral lines originating in Transition Region and low corona. DHs around ARs are seen as areas of reduced emission in many wavelengths, for instance in the SOHO/SUMER images in O V at 630 Å and S VI at 933 Å lines, as pointed out by Andretta et al. (2014), and in the SDO/AIA 171 Å waveband, as noted by Wang et al. (2011). DHs are very common solar features, yet we do not fully understand what their structure is, what determines their appearance, nor the physical mechanism that creates and sustains them. Moreover, because they are large-scale structures, of an extent comparable, if not larger, than that of the associated ARs, they may influence the irradiance of the Sun, especially during the maximum of the solar activity, when several ARs are present on the solar disk. Furthermore, DHs are sometimes mistaken for Coronal Holes (CHs) when seen in the UV and EUV. Nevertheless, to date there has not been a quantitative characterization that allows a clear distinction between these two solar features. In particular, the connectivity of DHs with the outer solar atmosphere and the solar wind must be different from the one of CHs in a way that is not yet clear. In this work we report preliminary results of the analysis of a comprehensive set of imaging and spectroscopic observations of the DH around AR 12706, observed on the solar disk on 22 April 2018, by using IRIS full-disk mosaics and SDO/AIA images, that together allow a tomography of the DH from the chromosphere up to the low corona. The first results highlight the main observational characteristics (line intensities, Mg II h & k fibrils) that can help distinguish between DHs and CHs. These characteristics can be used in future works aimed at systematically studying DHs.
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Abstract: One of the goals of Space Weather studies is to achieve a better understanding of impulsive phenomena, such as Coronal Mass Ejections (CMEs), in order to improve our ability to forecast them and reduce the risk to our technologically driven society. To do this, it is crucial to assess the application of theoretical models or even to create models that are entirely data-driven.The quality and availability of suitable data are of paramount importance. We have already merged public data about CMEs from both in-situ and remote instrumentation in order to build a database (DB) of CME properties. To evaluate the accuracy of such a DB and confirm the relationship between in-situ and remote observations, we have employed the drag-based model (DBM). DBM is an analytical model that assumes the aerodynamic drag caused by the surrounding solar wind to be the primary factor in the interplanetary propagation of CMEs. Here, we explore the parameter space for the drag parameter and solar wind speed using a Monte Carlo approach to analyse how well the DBM described the propagation of CMEs. With this method, we validate and/or correct the initial hypotheses about solar wind speed, and also yield additional information about CMEs. Using a data-driven approach, this procedure allows us to present a homogeneous, reliable, and robust dataset for the investigation of CME propagation.
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Composition of plasma in the solar corona is a tracer of the flow of plasma and energy from the solar interior. A complete understanding of coronal abundances not only provides us another perspective to look at complex processes such as wave propagation, wave absorption, convection, reconnection, and reconfiguration of magnetic fields and coronal heating, but it also has significant implications for solar-like stars. The method to parameterise and study coronal elemental abundances, is to use the first ionisation potential (FIP) bias, defined as taking the ratio of an element’s coronal to photospheric abundance with respect to H. In this seminar, we introduce FIP bias as a proxy to understand processes in different solar structures, ranging from small brightenings to active regions, using a wide range of techniques such as extreme-ultraviolet and radio imaging, and spectroscopy to study to the evolution of these solar structures, and interpret the physical processes underneath.
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Coronal magnetic fields are extrapolations of the observed photospheric field based on certain physical assumptions. One crucial parameter that is difficult to constrain observationally is , which dictates the twist of a field line in the corona. A few recent works e.g.1,2 are able to constrain through tracing the projected position of coronal loops as observed in extreme ultraviolet (EUV) images, thus a magnetic model is constructed where the field lines are constrained by both the photospheric field and the field line distribution in the corona. These studies are limited to a small number of field lines in active regions, where the coronal loops are clearly seen in images, and can be traced (manually, or automatically). I present here a novel image processing method, applied to EUV image time series, that reveals coherent and continuous faint motions everywhere in the corona 3 . These are signatures of slow magnetic waves. A modified Lucas-Kanade algorithm results in a mapping of the wave velocity field across broad regions of the low corona. The waves must follow the underlying magnetic field, thus the velocity field lines are a proxy for the magnetic field line distribution, as seen in projection by the observer. I present further evidence, based on a comparison with the distribution of the photospheric network, and with a potential field magnetic model, that the velocity field is closely related to the magnetic field 4 . Thus, in the near future, we plan to use maps of the wave velocity field, combined with photospheric field measurements, to build magnetic field extrapolations where is directly constrained by EUV observations across broad regions of the corona - including active regions, coronal holes, and the elusive quiet Sun. This approach will enable observationally-constrained magnetic models to be built routinely and efficiently, and promises a major breakthrough in probing the magnetic environment of the solar atmosphere.
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The solar corona has been investigated in the last decades through observations coming from several spacecraft. The Metis coronagraph, aboard the ongoing Solar Orbiter mission, extends the UVCS/SOHO spectrocoronagraph observations of the scattered ultraviolet emission of the coronal plasma performed during solar activity cycle 23, by simultaneously imaging the coronal visible light polarized brightness (VL pB), in the spectral bandpass 580-640 nm, and the coronal ultraviolet H I Lyα emission, in the spectral window 121.6 ± 10 nm. We present here some specific observations, such as those taken on May 15, 2020, from which detailed information about the coronal outward velocity were inferred by applying the Doppler dimming technique. Other results on the coronal solar wind outflow velocity were obtained by considering the quadrature of Solar Orbiter and PSP with respect to the Sun, when the same parcel of plasma was observed remotely with Metis between 3.5 R☉ and 6.3 R☉ on January 17, 2021, on the East limb and in situ by PSP at about 22 R☉ on January 18, 2021. In this case, information on several coronal parameters were obtained with unprecedented details, thanks to the high quality of Metis and PSP data. Finally, other results concern the first CME observed with Metis on January 16-17, 2021, during low-cadence synoptic mode observations. In this case, also considering data coming from instruments onboard other spacecraft and on Solar Orbiter, a 3D reconstruction and physical information of this structure were obtained. Therefore, Metis, even when operates in synoptic mode and in synergistic coupling with other instruments, allows to get novel and detailed information on the structure of the solar corona with an accuracy never reached until now.
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The solar atmosphere undergoes several changes during the solar cycle. Those changes can be identified by observing several solar features and their respective dynamics, which can be observed in many different wavelengths, and by analyzing the variations in the total solar irradiance. The latter is characterized by an increase in the total solar irradiance during the solar maximum and a decrease in the decay phase. In this talk, we will present the intensity variations of the Mg II h&k lines emission peak, observed by IRIS, under quiet sun conditions close to the disk center over the decaying phase of the solar cycle. The observed variations will then be interpreted and put in the context of the solar atmospheric physical conditions.
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The solar heating problem involves a coupled system (including the photosphere and chromosphere), thus requiring its study as a whole system. This requirement often leads to the combination of observations with different instrumental effects. One important missing piece in this context is the capability to observationally constrain the various physical mechanisms that heat the outer layers as predicted from numerical simulations. In this contribution we present the implementation of a newly-developed inversion algorithm that expands the capabilities of the NLTE inversion code STiC to better handle inversions combining multi-resolution observations, which can provide more constrained physical inferences. After we have checked its performance, we apply this inversion method to a quiet-Sun area co-observed with the SST (CRISP and CHROMIS) and IRIS spectrograph. After inferring the various physical properties of the atmosphere we compute the radiative losses from different sources and discuss their potential physical origin.
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At present, in connection with the intensive development of instruments for observing stars and exoplanets, the problem of analyzing the possible habitability of exoplanets is very actual. One of the important factors of auspicious conditions for the emergence of life are favorable radiation conditions. In talk we present various aspects of the space weather near exoplanets and in particular the occurrence of strong flares on host stars, which can prevent the formation of biological structures.
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Many magnetic structures in the solar atmosphere evolve rather slowly so that they can be assumed as (quasi-)static or (quasi-)steady and represented via magneto-hydrostatic (MHS) or magneto-hydrodynamic (MHD) equilibria, respectively. While exact 3D solutions would be desired, they are extremely difficult to find in steady MHD. We construct solutions with magnetic and flow vector fields that have three components depending on all three coordinates. We show that the non-canonical transformation method produces quasi-3D solutions of steady MHD by mapping 2D or 2.5D MHS equilibria to strongly related steady-MHD states, displaying highly complex currents. The existence of geometrically complex 3D currents within symmetric field-line structures provide the base for efficient dissipation of the magnetic energy in the solar corona by Ohmic heating. We also discuss the possibility of achieving force-free fields, and find that they only arise under severe restrictions of the field-line geometry and of the magnetic flux density distribution.
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Space weather phenomena have long captured the attention of the scientific community and, along with the recent technological development, the awareness that such phenomena can (and do) interfere with human activity on Earth has grown considerably. Coronal Mass Ejections (CMEs), together with solar flares and solar energetic particle events, are one of the main drivers of space weather. Therefore developing tools to provide information on their arrival at Earth’s nearby space became increasingly important. Liu et al. (2018) developed the CAT-PUMA tool, to obtain a fast and more accurate prediction of CME transit times. CAT-PUMA employs Support Vector Machine technique, based on a regression algorithm. In this work, we investigate further the potential of a Machine Learning approach to study the arrival of CMEs on Earth and explore its limitations. Furthermore, we present a new application of CAT-PUMA, employing Supervised Learning to obtain vital information about the arrival of CMEs at Earth. We trained three models, Support Vector Machine, Random Forest and Extreme Gradient Boosting (XGBoost), to obtain predictions on the Geo-effectiveness and arrival time of CMEs at Earth and compared their performance. The results show that supervised learning models can achieve a very promising performance, but are nevertheless dependent on evaluation metrics. Relying on optimistic evaluation criteria leads to models capable of producing transit time predictions with an error of 7 hours; while considering more conservative criteria, errors do not fall below 10 hours. Finally, we apply an interpretive approach to the proposed models in an attempt to extract as much information as possible from this study about the capabilities of Machine Learning in predicting the arrival of CMEs on Earth and possibly complement the predictions to help enhance the real-time estimation of the threat posed by CMEs.
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The initial evolution phase of a CME flux rope (FR) yields crucial information about their space weather impacts. Thus, we study this early phase by employing data-driven simulations, which have shown to be well-suited to model the low coronal FR evolution. The extraction of relevant field lines of the forming and (potentially) erupting structure is not trivial, however. Therefore, we develop a semi-automatic extraction and tracking scheme for CME flux ropes for simulation data. The extraction procedure makes use of the twist parameter in combination with mathematical morphology algorithms. We subsequently apply it to time-dependent data-driven magnetofrictional modelling (TMFM) results of active region AR12473. In particular, we investigate the evolution of FR properties and stability in relation to the data-driving nature of the simulations.
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Magnetic fields on small scales are ubiquitous in the Universe. Although they can often be observed in detail, their generation mechanisms are not fully understood. One possibility is the so-called small-scale dynamo (SSD). Prevailing numerical evidence, however, appears to indicate that an SSD is unlikely to exist at very low magnetic Prandtl numbers (PrM) such as those that are present in the Sun and other cool stars. Here we have performed high-resolution simulations of isothermal forced turbulence using the lowest PrM values achieved so far. Contrary to earlier findings, the SSD not only turns out to be possible for PrM down to 0.0031 but also becomes increasingly easier to excite for PrM below about 0.05. We relate this behaviour to the known hydrodynamic phenomenon referred to as the bottleneck effect. Extrapolating our results to solar values of PrM indicates that an SSD would be possible under such conditions.
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We use an image processing method called Mathematical Morphology (MM) to pinpoint different types of solar features (e.g., sunspots, faculae, coronal jets, flux rope structures, etc.) suspected to be responsible for strong solar activity and geoeffective eruptions. We validate the MM method by comparing the sunspot areas defined by the MM contours with other existing and reliable sunspot area catalogues (e.g., the hand-drawn Debrecen Heliophysical Observatory (DHO) catalogue and the cross-calibrated Mandal et al. (2020) catalogue). We can then apply the MM method to the identification of more complex features. For instance, we find the so-called delta-sunspots by overlaying Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) magnetograms and intensity images. However, the MM method also has limitations as it is affected by the strong effect of adverse weather conditions in solar images, and the fine-tuning of its parameters can only be done manually. In this way, Machine Learning (ML) techniques could prove to be very useful when combined with the MM method in what is called a Morphological Neural Network (e.g., Mondal et al. for de-raining images). The Neural Network complements the MM method by making it automatic and giving good performance metrics, while significantly reducing its number of parameters and providing some insight into the black-box model.
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Bunches of swaying spicules in the solar chromosphere exhibit a variety of complex dynamics that are clearly observed in the images of coronal hole regions. By calculating the line-of-sight integrated emission from three-dimensional radiative magnetohydrodynamic simulations, we obtain multiple episodes of rotation amongst clusters of spicules also reported in the sequence of high cadence observations on the solar limb. This perception of rotation, according to our findings, is associated with hot swirling plasma columns that we label as coronal swirling conduits (CoSCo). Some tall CoSCos seen in our simulations can potentially form by feeding on spicules and channeling this energy to the upper reaches of the solar atmosphere, even while the corresponding spicules fall back sun-ward.
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Coronal Mass Ejections (CMEs) are huge clouds of magnetised plasma expelled from the solar corona that can travel towards the Earth and cause significant space weather effects.The Drag-Based Model (DBM) describes the propagation of CMEs in an ambient solar wind as analogous to an aerodynamic drag.The drag-based approximation is popular because it is a simple analytical model that depends only on two parameters, the drag parameter $\gamma$ and the solar wind speed $w$. DBM thus allows us to obtain reliable estimates of CME transit time at low computational cost. Previous works proposed a probabilistic version of DBM, the Probabilistic Drag Based Model (P-DBM), which enables the evaluation of the uncertainties associated with the predictions. In this work, we infer the "a-posteriori" probability distribution functions (PDFs) of the $\gamma$ and $w$ parameters of the DBM by exploiting a well-established Bayesian inference technique: the Monte Carlo Markov Chains (MCMC) method. By utilizing this Bayesian method through two different approaches, an ensemble and an individual approach, we obtain specific DBM parameter PDFs for two ensembles of CMEs: those travelling with fast and slow solar wind, respectively. Subsequently, we assess the operational applicability of the model by forecasting the arrival time of CMEs. While the ensemble approach displays notable limitations, the individual approach yields promising results, demonstrating competitive performances compared to the current state-of-the-art, with a mean absolute error (MAE) of 9.86 ± 4.07 hours achieved in the best-case scenario.
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The ability to encode and simplify all information about the shape and distribution of data has turned Topological Data Analysis (TDA) into one of the most relevant fields in state-of-the-art data analysis. Among all the tools of TDA, persistent homology has proven to be one of the most relevant techniques, and has been applied in numerous fields of study, such as biomedicine, chemistry, atomic physics, or image classification. In this work, we study what persistent homology can offer in the analysis of solar magnetograms, with the purpose of providing a new tool that will serve as foundation for further studies of magnetic structures on the solar surface. We propose an approach based on the use of persistence diagrams belonging to various filtrations in order to be able to capture the whole magnetic scene involving a mixture of positive and negative polarities. We have applied the analysis to quiet sun and active regions observations, taken with both Hinode/SOT and SDO/HMI, respectively. Persistent diagrams have proven to be able to encode the spatial structure complexity of the magnetic flux of active regions by identifying the isolated and connected (interacting) structures. Holes or pores are also displayed in persistent diagrams, allowing as well for the identification of interacting structures of opposite polarities in the form of ring-like structures. The overall temporal evolution of active regions, as well as small scale events in quiet sun such as magnetic flux cancellation and emergence are also displayed in persistent diagrams and can be studied by observing the evolution of the diagrams and tracking the relevant features.
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I will give a brief historical introduction to experimental proofs of Einstein’s theory of general relativity. I will detail the attempts to detect gravitational waves with mass resonators and laser interferometers. After introducing LIGO (the Laser Interferometer Gravitational-wave Observatory detectors in the US), I will summarise the first detection, and all the science done with LIGO since the mid-2010s. I plan to talk about the contributions of our group - at Eotvos University in Budapest, Hungary - to the LSC (LIGO Scientific Collaboration). These include the development of data analysis techniques, a galaxy catalog to locate sources, modeling of astrophysical sources of gravitational waves, and the proposed development of a CubeSat fleet to better localize sources in the sky.
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Coronal Mass Ejections (CMEs) are huge clouds of magnetised plasma expelled from the solar corona that can travel towards the Earth and cause significant space weather effects.The Drag-Based Model (DBM) describes the propagation of CMEs in an ambient solar wind as analogous to an aerodynamic drag. The drag-based approximation is popular because it is a simple analytical model that depends only on two parameters, the drag parameter γ and the solar wind speed w. DBM thus allows us to obtain reliable estimates of CME transit time at low computational cost. Previous works proposed a probabilistic version of DBM, the Probabilistic Drag Based Model (P-DBM), which enables the evaluation of the uncertainties associated with the predictions. In this work, we infer the “a-posteriori” probability distribution functions (PDFs) of the γ and w parameters of the DBM by exploiting a well-established Bayesian inference technique: the Monte Carlo Markov Chains (MCMC) method. By utilizing this Bayesian method through two different approaches, an ensemble and an individual approach, we obtain specific DBM parameter PDFs for two ensembles of CMEs: those travelling with fast and slow solar wind, respectively. Subsequently, we assess the operational applicability of the model by forecasting the arrival time of CMEs. While the ensemble approach displays notable limitations, the individual approach yields promising results, demonstrating competitive performances compared to the current state-of-the-art, with a mean absolute error (MAE) of 9.86 ± 4.07 hours achieved in the best-case scenario.
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A new era of solar physics commences with observations of the quiet Sun using the 4-metre Daniel K. Inouye Solar Telescope/Visible Spectropolarimeter (DKIST/ViSP). We present full-Stokes observations taken during DKIST’s cycle 1, in the Fe I 630.1/630.2 nm lines, allowing us to examine small-scale magnetism in the photosphere. We use the Stokes Inversion based on Response functions (SIR) code to invert the Fe I line pair. We reveal the existence of a serpentine magnetic element for the first time. A statistical analysis is undertaken, comparing inversions of DKIST data with Hinode data. A novel machine learning technique is used to characterise and contrast the shapes of circular polarisation signals found in the ground-based and space-based data, and synthetic observations produced from MANCHA simulations are used to aid our understanding of the differences between datasets.
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Interactions between the highly dynamic atmosphere of our Sun and the magnetic fields permeating its atmosphere give rise to a wide variety of magnetohydrodynamic (MHD) wave phenomena. Combining observations of MHD waves with an applied mathematical description of the waveguides has allowed researchers to determine elusive physical quantities of the solar atmosphere using the methods of solar magneto-seismolgy. The ‘classical’ models utilised in this discipline describe straight, symmetrical MHD waveguides (slabs or flux tubes). A recent direction of research has focused on wave propagation in asymmetric slab waveguides, where the direction of propagation was strictly parallel to the magnetic field lines within the slab. Here, some further results are presented in the case when a magnetic slab is embedded in a non-magnetic, asymmetric environment, and the direction of propagation is allowed to deviate from the internal magnetic field lines of the slab. We describe this non-parallel wave propagation in various analytical approximations relevant to solar atmospheric waveguides (thin and wide slabs, low-beta plasmas) and present numerical solutions to the full dispersion relation to expand on our findings.
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The solar radiation in the cores of the Mg II h & k spectral lines strongly correlates with solar magnetic activity and global variations of magnetic fields with the solar cycle. This work provides a data-driven model of temporal evolution of the solar full-disk Mg II h & k profiles over the solar cycle. Based on selected 76 IRIS near-UV full-Sun mosaics covering almost the full solar cycle 24, we find the parameters of double-Gaussian fits of the disk-averaged Mg II h & k profiles and a model of their temporal evolution parameterized by the Bremen composite Mg II index. The Markov Chain Monte Carlo algorithm implemented in the IDL toolkit SoBAT is used in modeling and predicting temporal evolution of the Mg II h & k peak-to-center intensity ratio and the Bremen Mg II index. The relevant full-disk Mg II h & k calibrated profiles with uncertainties and spectral irradiances are provided as an online machine-readable table. To facilitate utilization of the model corresponding routines, written in IDL, are made publicly available at GitHub.
Co-authors: Stanislav Gunár (The Czech Academy of Sciences, Czech Republic), Pavol Schwartz (Slovak Academy of Sciences, Slovakia), Petr Heinzel (The Czech Academy of Sciences, Czech Republic; University of Wrocław, Poland), Wenjuan Liu (The Czech Academy of Sciences, Czech Republic)
Web announcement: https://espos.stream/2024/03/21/Koza/
Zoom link: https://zoom.us/j/165498165
(Meeting ID: 165 498 165)
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The solar atmosphere is filled with clusters of hot small-scale loops commonly known as Coronal Bright Points (CBPs). These ubiquitous structures stand out in the Sun by their strong X-ray and/or extreme-ultraviolet (EUV) emission for hours to days, which makes them a crucial piece when solving the solar coronal heating puzzle. Here we present a novel 3D numerical model using the Bifrost code that explains the sustained CBP heating for several hours. We find that stochastic photospheric convective motions alone significantly stress the CBP magnetic field topology, leading to important Joule and viscous heating concentrated around the CBP’s inner spine at a few megameters above the solar surface. We validate our model by comparing simultaneous CBP observations from SDO and SST with observable diagnostics calculated from the numerical results for EUV wavelengths as well as for the Halpha line using the Multi3D synthesis code.
Co-authors: Fernando Moreno-Insertis, Klaus Galsgaard, Kilian Krikova, Luc Rouppe van der Voort, Reetika Joshi, and Maria Madjarska
Web announcement: https://espos.stream/2024/04/04/Nobrega-Siverio/
Zoom link: https://zoom.us/j/165498165
(Meeting ID: 165 498 165)
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I briefly review some methods and measurements of elemental abundances in the solar atmosphere, with emphasis on the transition region and corona. Some limitations in the methods, in the modelling of the spectral line intensities, and in the observations are discussed. Examples from the X-rays, the EUV, the UV, the visible and near-infrared are presented. A significant improvement in the modelling some of the ions is being made available with CHIANTI version 11. All the observations indicate that the solar corona has photospheric abundances and that the hot 3 MK active region cores have stable enhancements of a factor of about 3.2 in the ratios of low to high-FIP elements. A lot of uncertainties and puzzles still exist, requiring further analyses and, more importantly, future instrumentation.
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We present the results of coordinated observations of the Swedish 1-m Solar Telescope with Solar Orbiter that took place from October 12th to 26th 2023. The campaign resulted in 7 datasets of various quality. The observational programs were adjusted to the seeing conditions. The observations cover two active regions and a coronal hole. We focus on the morphology and evolution of several targets that are observed from two vantage points. We share the lessons we learned and give an outline of our plans for October this year and the support we could give during remote sensing windows 16 and 17.
Web announcement:
https://espos.stream/2024/05/02/Danilovic/
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Zoom link: https://zoom.us/j/165498165
(Meeting ID: 165 498 165)
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Turbulence is a fundamental process that drives mixing and energy redistribution across a wide range of astrophysical systems. For warm (T ≈ 10^4K) plasma, the material is partially-ionised, consisting of both ionised and neutral species. The interactions between ionised and neutral species are thought to play a key role in heating (or cooling) of partially-ionised plasmas. Here mixing is studied in a two-fluid partially-ionised plasma undergoing the shear-driven Kelvin-Helmholtz instability to evaluate the thermal processes within the mixing layer. 2D numerical simulations are performed using the open-source (PIP) code that solves for a two-fluid plasma consisting of a charge-neutral plasma and multiple excited states of neutral hydrogen. Both collisional and radiative ionisation and recombination are included. In the mixing layer, a complex array of ionisation and recombination processes occur as the cooler layer joins the hotter layer, and vice-versa. In localised areas of the mixing layer, the temperature exceeds the initial temperatures of either layer with heating dominated by collisional recombinations over turbulent dissipation. The mixing layer is in approximate ionisation-recombination equilibrium, however the obtained equilibrium is different to the Saha-Boltzmann LTE equilibrium. The dynamic mixing processes may be important in determining the ionisation states, and with that intensities of spectral lines, of observed mixing layers.
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Observations of small-scale brightenings in the low solar atmosphere can provide valuable constraints on possible heating and heat transport mechanisms. We present a method for the detection and analysis of bright points (BPs), and demonstrate its application to time-series imagery of the Interface Region Imaging Spectrograph (IRIS) in the extreme ultraviolet. The method is based on spatio-temporal band-pass filtering, adaptive thresholding and centroid tracking, and records an event’s spatial position, duration, speed, total brightness, maximum brightness, and intrinsic brightness. Spatial area, brightness, and position are also recorded as functions of time throughout the event’s lifetime. Detected brightenings can fragment, or merge, over time – thus the number of distinct regions constituting a brightening event is recorded over time, and the maximum number of regions recorded as Nfrag, which is a simple measure of an event’s coherence or spatial complexity. The method is first tested on synthetic data based on Poisson statistics before being applied to real IRIS data. We present statistical characteristics of brightenings from the application of this method to 1330, 1400, and 2796 Å IRIS slit-jaw image time series. Several thousand events are recorded that coexist in all three channels, giving high confidence that they are real. Finally, we will also present continuing applications of this detection method to analyse a large set of BPs and their characteristics – over 12,000 BPs in total – and compare those that are found within ‘Active’ and ‘Quiet’ domains within a QS region, as well as possible future applications of the detection method.
Zoom link: https://zoom.us/j/165498165
(Meeting ID: 165 498 165)
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t is well understood that the dynamics of sunspots lead to energy being transferred to the solar atmosphere and stored in the coronal magnetic field. This provides a surplus of energy that may be released in solar eruptions. The driving mechanisms for this energy transfer may include sunspot rotations, both within individual sunspots and between sunspot pairs. Calculation of the rotations of individual sunspots have been carried out by several authors, but studies of the rotation of sunspot pairs has been less systematically investigated. Calculation of rotations in either case rely on careful tracking of the sunspots from observation to observation. Identification and tracking of sunspots is therefore essential to understanding the energies in play that lead up to solar eruptions. To date, this has predominantly been done manually which has restricted many studies to being a small number of case studies rather than large statistical samples. In order to construct large samples, the careful tracking of sunspots must be automated. We present a fully automatic method to identify and track sunspots in long sequences of data from the Solar Dynamics Observatory Helioseismic and Magnetic Imager (SDO/HMI) at a high temporal resolution. This includes registering the splitting and merging of sunspots, and allocating sunspots to active regions. This information can be fed into algorithms to measure the rotation of individual sunspots or used to calculate the relative motion of sunspots with respect to each other (including co-rotation). The method is applied to a four-month data set that has previously been analysed using a semi-automatic method where the basic sunspots were identified by eye, and the results are compared to determine any differences between the methods. From this data, sunspot dynamics such as sunspot rotation, shearing and merging are calculated, alongside sunspot pair interactions. Case studies of successfully tracked sunspots will be presented, showing examples of the individual sunspot rotations and some initial results involving sunspot pair interactions with correlations to solar activity.
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The prediction of geomagnetic storms is becoming more and more important, with the aim to take effective measures for avoiding the possible damage from the extreme events. One of the important parameters when modeling CMEs and CME-driven shocks, is their arrival time at Earth. We present a study of several halo CMEs with the propagation direction which significantly deviated from the Sun-Earth line and as a result, CMEs impacted Earth as flank-encounters. We modeled selected events with the default-setup of EUHFORIA and the Cone model for the CMEs. The aim of our study is to better understand the importance of the CME’s direction of propagation in the input parameters of the Cone model and improve the modeled arrival time at Earth. We selected events that were propagating strongly non-radialy in the low corona, in order to understand how important are the effects of the deflections in the low corona, in the direction of propagation. Our results show that, when the data from the DONKI database are used, the modeled arrival time has the largest discrepancy(≥10h) when compared with observations. When the input parameters are taken employing the GCS fitting technique though, up to the height of 12 Ro (solar radii), the accuracy of the modeled arrival time improves, shifting closer to the observed ones. This result reflects the characteristic that, up to the heights of about 10 Ro, CMEs experience all the low coronal deflections and have taken their final direction of propagation.
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