Geomagnetism and Aeronomy

Qualitative estimates of the dynamics of the stellar convective zone as a whole are useful either when observation - data on a star are scares or as a preliminary step before constructing a more complex model that requires complicated computations. In this work, we present a qualitative model that describes the evolution of the mean square values of the velocity and magnetic field in the convective zone of a Sun-like star. The stability of possible equilibrium values of the mean squares of velocity and magnetic field is investigated. Solutions of the model equations are obtained for different values of buoyancy and the ratio of the convection and magnetic field timescales. It is shown that the following scenarios are possible: 1) the magnetic field increases starting from an arbitrary small initial value; 2) the magnetic field vanishes being initially finite; 3) the velocity and magnetic-field behavior in the vicinity of the stationary values and far from them can differ significantly. The amplification/decay of the mean-square magnetic field does not depend on the initial conditions, but is determined solely by the parameters of the stellar convective zone. The parameters of the Sun’s convective zone correspond to a boundary case between scenarios 1 and 2, and their small changes can lead to different outcomes.

The nature of precursors of solar flares and their relationship with subsequent flares are still not clear. This is due, in particular, to the lack of systematic statistical work, the relatively incomplete studies of individual events (out of the context of evolution of the entire active region), and the ambiguity of the term “precursor” itself. In this paper, we consider the dynamics of the NOAA 12230 active region (AR), in which a series of homologous flares (C5-C9) occurred on December 9, 2014 within 12 hours with a mean occurrence rate of about 2 hours. This AR is characterized by rapidly increasing flare activity followed by a rapid decay, which can serve as a good example for studying potential precursors of a series of flares. We investigated the evolution of AR NOAA 12230 over a relatively long time interval (several days) and its transition from the ‘no-flare’ state to the flare-active regime. For this purpose, we studied the magnetic field dynamics using SDO/HMI vector magnetograms, UV-EUV images based on SDO/AIA data, and GOES/XRS and RHESSI X-ray data. Thus, we identified several phases in the AR evolution in terms of the magnetic-field dynamics and flare activity. A method for building hourly integral maps of UV variations based on AIA 1600 _A data was proposed. It was concluded that significantly increased variations in the chromospheric radiation against small soft X-ray and UV emission fluxes from the corona observed on December 8, 2014, together with the emergence of a magnetic flux, can be considered as a precursor to a series of flares. We also analyzed the appearance of X-ray sources of weak bursts before the series of flares. X-ray bursts developed in the same plasma structures where future flares occurred. The results obtained show the importance and prospects of using new methods of synoptic observations of the Sun for collecting statistics (“history”) of the energy release in AR in different ranges of the electromagnetic spectrum. In other words, it is important to monitor not only the dynamics of the magnetic field structure but also how the AR releases the stored magnetic energy. An integrated approach will allow the development of new methods for predicting flares, perhaps, more advanced than simply taking into account the magnetic field structure.

In this paper, we present and discuss the results of using the machine learning methods to compile a catalog of solar flares observed with the Siberian Radioheliograph (SRH). The high sensitivity of the instrument, as well as the use of time profiles of the sum of correlation coefficients of antenna pairs (correlation plots) for event searching allowed us to include in the catalog the weak events, which are poorly distinguished in the time profiles of the emission flux. We proposed and tested a technique for selecting candidate events that allows the onset, maximum, and end of a solar flare to be determined by analyzing the derivative of the time profile given by a numerical function. Since the purpose of the catalog was to select wideband events, we introduced a criterion that allows automatic event selection based on simultaneous responses at several frequencies. Support Vector Machine (SVM) method was used in the test mode to assert the solar origin of the events and specify the quality of observational data. The volume of observational data obtained by the SRH in the second half of 2023 and in 2024 provides extensive material for both training and testing the model. The method was applied to the time profiles obtained in the 9–10 GHz band to divide them into “flare”, “background”, and “artifact”.

The acceleration of quasi-thermal particles by an electric field, whose velocity is greater than the thermal velocity, is considered in various interpretations for the dynamic brake force of electrons arising due to Coulomb collisions. It is shown that if the velocity of electrons exceeds the thermal velocity by a factor of two, then the forces of dynamic braking, taking into account electron-electron collisions (Spitzer approximation), and changes in the distribution function of background electrons under the influence of an electric field (Dreicer approximation), practically coincide. If the electric field is much smaller than the Dreicer field, then the Spitzer and Harrison (accounting both for electron-electron collisions and changes in the background electron distribution function) approaches match to within a coefficient. The implications of these results for the acceleration of quasi-thermal electrons in solar flares are discussed.

The main feature of the last year of the development of the 25th solar cycle of the second cycle of the 2nd epoch of reduced solar activity is a sharp increase in spot-forming activity and, especially, the number of flare-active areas and, accordingly, an increase in the number of large solar flares, the most powerful in the growth branch of the current solar cycle. This placed it above the low solar cycle 24 and the transitional cycle 23 in terms of the number of significant flares during the corresponding period. In the maximum phase of the current solar cycle 25, the number of sympathetic flares of significant classes (> M1) increased noticeably, which made it possible to begin studying both such flares themselves and the active regions.

The interaction of non-thermal electrons injected into a flare loop and whistler turbulence in it is investigated. Turbulence generated by an external source is considered with spatial and temporal characteristics similar to those of electron injection; it is assumed that both of these processes occur at the same time and in the same place during energy release in a flash. The peculiarities of the non-thermal electron distributions transformation in energy and pitch angles are revealed, taking into account the reverse effect of electrons on the whistlers’ turbulence. It is established that the power of the turbulence source and the turbulent capture of non-thermal electrons significantly affect the process of additional acceleration. In contrast to the model with a given stationary distribution of Whistler turbulence, in the model with a consistent interaction, there is a significant decrease in the energy density of turbulence, which significantly reduces the efficiency of electron acceleration.