European Solar Physics Online Seminar Archive

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]

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