European Solar Physics Online Seminar Archive

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]