Vol 57, No 8 (2017)

Over the last decade, a number of papers discussing long-period sunspot oscillations—with periods of tens and hundreds of minutes—have appeared. These studies are mainly based on the data of space probes. Despite the many possibilities in the time resolution and the absence of a distorting effect of the atmosphere, such studies face some artifacts connected with the pixel structure of an image (Nagovitsyn and Rybak, 2014); the existence of this phenomenon itself is still under discussion. On the other hand, starting already in the 1970s, long-period oscillations were studied by groundbased methods, including optical investigations after the mid-1980s. Since the papers containing the results of this research were mainly published in the Russian literature, which is not readily available for a wide range of specialists (mostly, in the Solnechnye Dannye bulletin or the proceedings of all-Russia conferences), they have been lost. The objective of this review is to recall these studies and their results and to combine the earlier findings with up-to-date ones.

The magnetic properties of the shadow of magnetic-related leading and trailing spots (those connected by forces lines of magnetic field, which are calculated from a field in potential approximation) are studied in this work. The correlations are established between individual characteristics of the field in the spot shadow and these characteristics from the shadow area S for spot pairs, for which the minimum angle between the measured vector of magnetic induction B in the shadow of the leading (L) spot and positive normal to the solar surface is lower than in the trailing (F) spot (α_min-L < α_min-F) and, vice versa, when α_min-L > α_min-F. It is shown that the α_min-L(S_L), α_min-F(S_F), B_max-L(S_L) and B_max-F(S_F) correlations are similar behaviorally and quantitatively for two groups of spots with different asymmetries of a magnetically connected field (B_{max-L, F} is the maximum of magnetic induction in the shadow of leading and trailing spots). The correlation between the average angles within the spot shadow 〈α_{L, F}〉 and the area of the spot shadow S_{L, F} and between the average value of magnetic induction in the spot shadow 〈B_{L, F}〉 differ in two cases. In most studied spot pairs, the leading spot is closer to the dividing line of polarity between the spots rather than the trailing one.

This paper is devoted to an analysis of brightness temperatures of the polar and low-latitude coronal holes on the Sun in the cm-wave range during periods of minimum solar activity. Data from observations of the polar coronal hole received by the RATAN-600 radio telescope during the solar eclipse of March 29, 2006, and low-latitude observations of coronal holes and quiet Sun received earlier with the RATAN-600 and BPR radio telescopes in the period of minimum solar activity have been used in the paper. The obtained good agreement between the brightness temperatures of cm-wave emission of the polar coronal hole above the North Pole of the Sun and of the low-latitude coronal holes against the background of the quiet Sun reveals the identity of the temperature properties of large coronal holes, irrespective of the mode of their arrangement and location on the Sun during the periods of minimum solar activity.

The heliospheric current sheet (HCS) is modified by the solar activity. HCS is highly inclined during solar maximum and almost confined with the solar equatorial plane during solar minimum. Close to the HCS solar wind parameters as proton temperature, flow speed, proton density, etc. differ compared to the region far from the HCS. The Earth’s magnetic dipole field crosses HCS several times each month. Considering interplanetary coronal mass ejections (ICME) and high speed solar wind streams (HSS) free periods an investigation of the HCS influence on the geomagnetic field disturbances is presented. The results show a drop of the Dst index and a rise of the AE index at the time of the HCS crossings and also that the behavior of these indices does not depend on the magnetic polarity.

The ordinary mode of gyrosynchrotron radiation was identified to be predominant in some segments of flare loops in solar flares of July 19, 2012, and October 22, 2014. These events were studied by investigation of the quasi-transverse propagation effect on the observed polarization. The analysis involved reconstruction of the magnetic field topology at the linear force-free approximation based on the data of the SDO HMI space telescope and the subsequent simulation of radio emission of flare loops with the GX Simulator software package. The quasi-transverse propagation effect was established to be characteristic for both events, but its influence on the radio emission polarization at a frequency of 17 GHz was observed only in the October 22, 2014 flare.

The influence of whistlers on the distribution and gyrosynchrotron radiation of fast electrons injected into a coronal magnetic trap is considered. The kinetic equation in the Fokker–Planck approximation with consideration of fast electron scattering, both on background plasma particles and on whistlers, is solved for an inhomogeneous trap. It is supposed that the source of whistlers is a nonstationary process of flare energy release. Having found the fast electron distribution, we can calculate their gyrosynchrotron microwave emission. The influence of nonthermal electron scattering on whistlers are compared with the effects of scattering on Coulomb collisions. It is shown that whistlers considerably modify the emission characteristics of a loop at a certain energy density; in particular, they steepen the frequency spectrum. This is useful for microwave diagnostics of plasma turbulence in the flare loop.

Several new models of solar prominences with helical geometry of the magnetic field were suggested; the possibility of the construction of a filament model with a fine filament structure was shown. For the first time, numerical solutions of the magnetohydrostatical problem, in which a fine structure of an equilibrium solar filament is expressed not only in the vertical direction but also in the direction along the filament axis, were obtained.

A three-dimensional simulation of the microwave emission of nonthermal electrons within a flare magnetic loop that takes into account the influence of the chromosphere has been carried out. The paper investigates the possibility of the generation of a microwave spectrum (observed for some solar flares) with a maximum in the centimeter wavelength range and a positive slope in the millimeter one under the some distribution of the magnetic field strength and the parameters of the anisotropic nonthermal electrons along the loop. By the example of the event on July 5, 2012, it is shown that nonthermal electrons can be responsible not only for the centimeter bell-shaped emission spectrum generated in the coronal part of the loop but also for the increasing millimeter spectrum generated in the chromosphere.

The properties of the 12-h artifact in the data of the SDO/HMI instrument (Helioseismic and Magnetic Imager) caused by the nonzero radial velocity of the station relative to the Sun are investigated. The study has been carried out with respect to long-period oscillations of the magnetic field of sunspots for different station positions in the Earth’s orbit by the alternative spectral method of singular decomposition of the signal CaterPillarSSA. Features of artifact filtering, both in special positions of the station (at the points of aphelion and perihelion) and at arbitrarily selected orbital points, are considered. It is shown that the 12-h artifact mode can be completely filtered from the time series of the observed variable, not only at these two orbital points (because of the symmetry of the station’s radial velocity with respect to the zero mean here) but also at any others. It is shown that only a 12-h mode is physically justified, while the 24-h harmonic appears only as an artifact in the Fourier decomposition of the amplitude-modulated signal. It is emphasized that the values of the magnetic field measured with SDO/HMI are sensitive only to the station’s radial velocity absolute values with respect to the Sun and do not depend on its direction. It has been noted that the periods of sunspot oscillation as a whole obtained from SDO/HMI data after orbital artifact filtration fit well into the dependence diagram of the period of sunspot oscillations on the value of its magnetic field strength constructed earlier by SOHO/MDIdata.

The energy potential of solar nanoflares is estimated with a new approach proposed by the author. This approach is based on the drift mechanism for the formation of a dense loop structure in the magnetic field of a bipolar source. The densification process is assumed to proceed until the appearance of unmagnetized protons. These protons produce a current that heats the loop structure. The presence of bipolar sources is associated with local amplification of the background magnetic field by mesogranulation cells. The calculations conducted with the proposed model, which take into account observational data, yield a nanoflare energy range of 10²⁴–10²⁶ erg. The same estimates are obtained from the observed emission of nanoflare radiation. This fact is evidence, on the one hand, that the proposed model is adequate to the given process and, on the other hand, that there are no significant fluxes of the energy of this process as thermal conductivity and nonthermal particle beams. This situation is characterized by a maximum possible nanoflare energy release at a level of ≈10²⁷ erg during the mesogranule lifetime (≈10⁴ s), which yields an intensity of the energy flux of ≈10⁵ erg/s cm². This flux is insufficient to heat even the quiet regions of solar corona.

Macroscopic discontinuous structures observed in the solar wind are considered in the framework of magnetic hydrodynamics. The interaction of strong discontinuities is studied based on the solution of the generalized Riemann–Kochin problem. The appearance of discontinuities inside the magnetosheath after the collision of the solar wind shock wave with the bow shock front is taken into account. The propagation of secondary waves appearing in the magnetosheath is considered in the approximation of one-dimensional ideal magnetohydrodynamics. The appearance of a compression wave reflected from the magnetopause is indicated. The wave can nonlinearly break with the formation of a backward shock wave and cause the motion of the bow shock towards the Sun. The interaction between shock waves is considered with the well-known trial calculation method. It is assumed that the velocity of discontinuities in the magnetosheath in the first approximation is constant on the average. All reasonings and calculations correspond to consideration of a flow region with a velocity less than the magnetosonic speed near the Earth–Sun line. It is indicated that the results agree with the data from observations carried out on the WIND and Cluster spacecrafts.

Solar flare prediction remains an important practical task of space weather. An increase in the amount and quality of observational data and the development of machine-learning methods has led to an improvement in prediction techniques. Additional information has been retrieved from the vector magnetograms; these have been recently supplemented by traditional line-of-sight (LOS) magnetograms. In this work, the problem of the comparative prognostic efficiency of features obtained on the basis of vector data and LOS magnetograms is discussed. Invariants obtained from a topological analysis of LOS magnetograms are used as complexity characteristics of magnetic patterns. Alternatively, the so-called SHARP parameters were used; they were calculated by the data analysis group of the Stanford University Laboratory on the basis of HMI/SDO vector magnetograms and are available online at the website (http://jsoc.stanford.edu/) with the solar dynamics observatory (SDO) database for the entire history of SDO observations. It has been found that the efficiency of large-flare prediction based on topological descriptors of LOS magnetograms in epignosis mode is at least s no worse than the results of prognostic schemes based on vector features. The advantages of the use of topological invariants based on LOS data are discussed.

A solar faculae having an appearance of quite long-lived magnetic nodes can perform (as well as sunspots, chromospheric filaments, coronal loops) free oscillations, i.e., they can oscillate about the stable equilibrium position as a single whole, changing quasi-periodically magnetic field averaged over the section with periods from 1 to 4 hours. Kolotkov et al. (2017) described the case in which the average magnetic field strength of the facula node considerably decreased during observations of SDO magnetograms (13 hours), and, at the same time, its oscillations acquired a specific character: the fundamental mode of free oscillations of the facula considerably increased in amplitude (by approximately two times), while the period of oscillations increased by three times. At the end of the process, the system dissipated. In this work, we present the exact solution of the equation of small-amplitude oscillations of the system with a time-variable rigidity, describing the oscillation behavior at which the elasticity of the system decreases with time, while the period and amplitude of oscillations grow.