European Solar Physics Online Seminar Archive

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]

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. [more]

Minifilaments are miniature versions of filaments, first observed in H-alpha filtergrams of quiet Sun. Recent studies have showcased their association with small-scale eruptive events, highlighting their importance in energetic processes of the quiet Sun. We present the first detailed study of such an event, using high-cadence, high-spectral resolution imaging observations. The minifilament formed between small-scale, opposite-polarity magnetic concentrations and erupted within an hour after its appearance in H-alpha, exhibiting a twisted, thread-like structure. Its eruption took place in two phases (slow and fast), producing a coronal dimming, while part of the erupting material returned to the chromosphere. The observed similarities to large-scale filament eruptions indicate the action of common mechanisms. Their properties, combined with their abundance in quiet Sun, constitute minifilaments ideal targets for the new ground-based solar telescopes. [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]

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 (<M5-class flares) active regions (ARs) using the Space-weather Helioseismic Magnetic Imager Active Region Patches vector magnetic field data. Each of the selected ARs contain a δ-type spot. The three helicity components of these ARs were analyzed using wavelet analysis. Localized peaks of the wavelet power spectrum (WPS) were identified and statistically investigated. We find that (i) the probability density function of the identified WPS peaks for all the EM/SH/T profiles can be fitted with a set of Gaussian functions centered at distinct periods between ∼3 and 20 hr. (ii) There is a noticeable difference in the distribution of periods found in the EM profiles between the flaring and nonflaring ARs, while no significant difference is found in the SH and T profiles. (iii) In flaring ARs, the distributions of the shorter EM/SH/T periods (<10 hr) split up into two groups after flares, while the longer periods (>10 hr) do not change. (iv) When the EM periodicity does not contain harmonics, the ARs do not host a large energetic flare. (v) Finally, significant power at long periods (∼20 hr) in the T and EM components may serve as a precursor for large energetic flares. [more]

[more]

[more]