European Solar Physics Online Seminar Archive

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. [more]

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. 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. [more]

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. 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. [more]

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. [more]

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. [more]

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. 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. [more]

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. [more]

The Mg I b2 line at 5173 Å forms over a large range of heights but itscore, which forms under conditions of non-local thermodynamicequilibrium, is most sensitive to heights near the temperature minimum,a region of the solar atmosphere that has not been sufficientlyexplored. The next-generation solar observatories will have access tothis spectral line and will allow for multi-line observations to studythe different layers of the solar atmosphere simultaneously and withunprecedented polarimetric sensitivity. We will present a morphologicalclassification of the intensity and circular polarization profiles ofthis spectral line at high-spatial-resolution, using observations fromthe Swedish 1-m Solar Telescope. We will also discuss the results of theweak field approximation applied to the Mg I b2 line, and theircomparison with inversion results of the Fe I 6173 Å line to understandhow the magnetic field changes with height in the solar atmosphere. [more]

The Mg I b2 line at 5173 Å forms over a large range of heights but itscore, which forms under conditions of non-local thermodynamicequilibrium, is most sensitive to heights near the temperature minimum,a region of the solar atmosphere that has not been sufficientlyexplored. The next-generation solar observatories will have access tothis spectral line and will allow for multi-line observations to studythe different layers of the solar atmosphere simultaneously and withunprecedented polarimetric sensitivity. We will present a morphologicalclassification of the intensity and circular polarization profiles ofthis spectral line at high-spatial-resolution, using observations fromthe Swedish 1-m Solar Telescope. We will also discuss the results of theweak field approximation applied to the Mg I b2 line, and theircomparison with inversion results of the Fe I 6173 Å line to understandhow the magnetic field changes with height in the solar atmosphere. [more]

The X1.6 flare observed on 22 October 2014 (SOL2014-10-22T14:28) was among the strongest flares that occurred in the magnetically complex, great active region NOAA 12192. Despite the large amount of released energy, it was a confined flare, without an accompanying CME. In our work we attempt to deepen our understanding of the magnetic field configuration of the active region NOAA 12192. We analyzed the polarization signatures during the flare using full spectro-polarimetric data acquired by the IBIS/DST instrument along the photospheric Fe I 617.3 nm and the chromospheric Ca II 854.2 nm lines in a one-hour time interval immediately following the peak of the X1.6 flare. The results obtained provide evidence of significant changes in the magnetic field configuration of the chromosphere during the analyzed time interval. [more]

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. [more]

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. [more]

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. [more]

The physical conditions resulting in the formation and disappearance of penumbral regions are poorly understood. We investigated these conditions by using high-resolution spectropolarimetric observations of a sunspot penumbra from different instruments at ground- and space-based telescopes, namely the SST/CRISP, SDO/HMI, and Hinode/SP. The studied data allowed us to assess the evolution of the magnetic and velocity properties of plasma in the observed region and to analyze the role of several processes found therein. The penumbra forms only on one side of the observed region, characterized by the absence of an overlying magnetic canopy. The penumbra later disappears progressively in time and space. This final evolution of the studied region seems to be governed by the presence of moving magnetic features (MMFs) and of overlying canopies. [more]