L. V. Yasnov

The Growth rate of upper-hybrid waves and dynamics of microwave zebra structures

The growth rate of the upper-hybrid waves with different velocities of superthermal electrons is computed considering a finite temperature of the background plasma and relativistic corrections. Based on these computations two examples of high-frequency zebra structures are interpreted. The sequence of the continuum, zebra structure, and continuum observed in the 29 October 2000, event is explained as an increase and following decrease of the velocity of superthermal electrons in the range of v=0.1–0.3 c. On the other hand, the zebra structure observed during the 18 March 2003 event represents an example with fast electron acceleration.

The altitude structure of the coronal magnetic field of AR 10933

The magnetic fields of solar active regions are analyzed using a method based on comparing the spatial structures of the reconstructed magnetic field and of the radio emission of the active region. Two approaches are used: comparing the radio size of the active region and the corresponding size calculated using the reconstructed magnetic field, and comparing the radio spectra that are observed and calculated using the reconstructed magnetic field. Overall, the calculated sizes and spectra correspond fairly well to the observational data, making it possible to estimate physical parameters of the emitting region, such as the electron density and temperature.

About the nature of long-term microflare energy release in solar active regions

In this paper we compare the long-term impulsive microflares (microbursts (MB) in the range 1000 MHz) using RATAN-600 observations and impulsive noise storm (NS) emission on frequencies 233 and 164 MHz using Nancay radioheliograph data. It is shown that these processes result from destruction of current sheets at the top of a coronal loop as a result of the development of plasma instability sequences. The coincidence in time between the generation of microbursts and coronal mass ejections is of special interest. The impulses of NS radio emission at meter waves arise after the appearance of the MB at decimeter waves. The temporal parameters of the processes are determined. It is shown that the MB duration is determined by the spreading time of the magnetic flux in the reconnection region along the current sheet. The flux of energy in the form of fast electrons is equal to 9.2 x 10 4 ergs cm -2 s -1 .

Temperature dependent growth rates of the upper-hybrid waves and solar radio zebra patterns

Context. The zebra patterns observed in solar radio emission are very important for are plasma diagnostics. The most promising model of these patterns is based on double plasma resonance instability, which generates upper-hybrid waves, which can be then transformed into the zebra emission. Aims. We aim to study in detail the double plasma resonance instability of hot electrons, together with a much denser thermal background plasma. In particular, we analyse how the growth rate of the instability depends on the temperature of both the hot plasma and background plasma components. Methods. We numerically integrated the analysed model equations, using Python and Wolfram Mathematica. Results. We found that the growth-rate maxima of the upper-hybrid waves for non-zero temperatures of both the hot and background plasma are shifted towards lower frequencies comparing to the zero temperature case. This shift increases with an increase of the harmonic number s of the electron cyclotron frequency and temperatures of both hot and background plasma components. We show how this shift changes values of the magnetic eld strength estimated from observed zebras. We confirmed that for a relatively low hot electron temperature, the dependence of growth rate vs. both the ratio of the electron plasma and electron cyclotron frequencies expresse distinct peaks, and by increasing this temperature these peaks become smoothed. We found that in some cases, the values of wave number vector components for the upper-hybrid wave for the maximal growth rate strongly deviate from their analytical estimations. We confirmed the validity of the assumptions used when deriving model equations.

Determination of plasma parameters in solar zebra radio sources

Aims. We present a new method for determining the magnetic field strength and plasma density in the solar zebra radio sources. Methods. Using the double plasma resonance (DPR) model of the zebra emission, we analytically derived the equations for computing the gyroharmonic number s of selected zebra lines and then solved these equations numerically. Results. The method was successfully tested on artificially generated zebras and then applied to observed ones. The magnetic field strength and plasma density in the radio sources were determined. Simultaneously, we evaluated the parameter Lnb = 2Lb/(2Ln − Lb), where Ln and Lb are the characteristic scale-heights of the plasma density and magnetic field strength in the zebra source, respectively. Computations show that the maximum frequency of the low-polarized zebras is about 8 GHz, in very good agreement with observations. For the high-polarized zebras, this limit is about four times lower. Microwave zebras are preferentially generated in the regions with steep gradients of the plasma density, such as in the transition region. In models with smaller density gradients, such as those with a barometric density profile, the microwave zebras cannot be produced owing to the strong bremsstrahlung and cyclotron absorptions. We also show that our DPR model is able to explain the zebras with frequency-equidistant zebra lines.

On Properties of Microwave Sources Located above the Neutral Line of Radial Magnetic Field

In this study we continue our investigation of the radio sources located above the neutral line of the radial magnetic field in solar active regions, i.e., the so-called neutral line associated sources (NLS). The nature of NLS is still far from being understood. To study it, we use the spectroscopic capabilities of the new broadband polarimetric facility of the RATAN-600 radio telescope. We study the radio spectra of NLS sources in several solar active regions over a wide range of variations of their sizes. We find the NLS radio emission fluxes to be related to the gradient of the quasi-longitudinal magnetic field in the photosphere. We estimate the vertical positions of NLS relative to the cyclotron radio sources. We find fine spectral features in the NLS emission, which confirm the presence of a current sheet in their sources. We associate the appreciable lack of polarization in such sources with their location near the tops of the coronal arches.

Physical conditions in the low corona and chromosphere of solar active regions according to spectral radar measurements

The physical conditions in the low corona and chromosphere of solar active regions are studied. A diagnostics technique based on multiwave observations in the centimeter range, photospheric magnetic field extrapolation, and radioemission calculations has been applied. The calculated spatial and spectral structure of the radioemission has been compared with RATAN-600 spectral-polarization observations with a high spatial resolution. The effect of the plasma physical parameters on the emission structure character in a complex magnetic field topology in active regions is analyzed. Modeling of the spectral singularities at a quasi-periodic propagation of the radioemission is presented.

On group velocity delays in microwave millisecond oscillating events and radio spikes

Time delays between opposite polarizations, interpreted by a dierence between the ordinary and extraordinary modes, are studied. Two dierent density and magnetic field models are used, and three emission mechanisms generating radio waves in double upper-hybrid, plasma, and gyro frequencies are considered. It is found that time delays and their spectra can reach various values and forms depending on plasma parameters in the radio wave emission and propagation. For the emission in double upper-hybrid frequency the considerable decrease of the time delay is due to an increase of the electron plasma density in the radio wave generation. In a dense flare plasma the power index a of the time delay spectrum is negative. On the other hand, in a diluted plasma a becomes positive, but in this case it is much lower than observed for dm-pulsations (a= 3). Furthermore model values are compared with the time delays presented by Fleishman et al. (2002), and it is shown that the best agreement is with the model assuming the emission on double gyro-frequency. But, in this case the gyro-resonant absorption limits the angle for escape of radio waves to< 3-10. Finally, an eect of the dierence of the group and light velocities on the frequency drift is analyzed. It is shown that such an eect can be important for wave propagation along magnetic field lines at frequencies close to the plasma frequency.

Features of Microwave Radiation and Magnetographic Characteristics of Solar Active Region NOAA 12242 Before the X1.8 Flare on December 20, 2014

This paper continues the cycle of authors’ works on the detection of precursors of large flares (M5 and higher classes) in active regions (ARs) of the Sun by their microwave radiation and magnetographic characteristics. Generalization of the detected precursors of strong flares can be used to develop methods for their prediction. This paper presents an analysis of the development of NOAA AR 12242, in which an X1.8 flare occurred on December 20, 2014. The analysis is based on regular multiazimuth and multiwavelength observations with the RATAN-600 radio telescope in the range 1.65–10 cm with intensity and circular polarization analysis and data from the Solar Dynamics Observatory (SDO). It was found that a new component appeared in the AR microwave radiation two days before the X-flare. It became dominant in the AR the day before the flare and significantly decreased after the flare. The use of multiazimuth observations from RATAN-600 and observations at 1.76 cm from the Nobeyama Radioheliograph made it possible to identify the radio source that appeared before the X-flare with the site of the closest convergence of opposite polarity fields near the neutral line in the AR. It was established that the X-flare occurred 20 h after the total gradient of the magnetic field of the entire region calculated from SDO/HMI data reached its maximum value. Analysis of the evolution of the microwave source that appeared before the X-flare in AR 12242 and comparison of its parameters with the parameters of other components of the AR microwave radiation showed that the new source can be classified as neutral line associated sources (NLSs), which were repeatedly detected by the RATAN-600 and other radio telescopes 1–3 days before the large flares.This paper continues the cycle of authors works on the detection of precursors of large flares (M5 and higher classes) in active regions (ARs) of the Sun by their microwave radiation and magnetographic characteristics. Generalization of the detected precursors of strong flares can be used to develop methods for their prediction. This paper presents an analysis of the development of NOAA AR 12242, in which 1 ar X iv :1 71 1. 09 13 4v 1 [ as tr oph .S R ] 2 4 N ov 2 01 7 an X1.8 flare occurred on December 20, 2014. The analysis is based on regular multiazimuth and multiwavelength observations with the RATAN-600 radio telescope in the range 1.65-10 cm with intensity and circular polarization analysis and data from the Solar Dynamics Observatory (SDO). It was found that a new component appeared in the AR microwave radiation two days before the X-flare. It became dominant in the AR the day before the flare and significantly decreased after the flare. The use of multiazimuth observations from RATAN600 and observations at 1.76 cm from the Nobeyama Radioheliograph made it possible to identify the radio source that appeared before the X-flare with the site of the closest convergence of opposite polarity fields near the neutral line in the AR. It was established that the Xflare occurred 20 h after the total gradient of the magnetic field of the entire region calculated from SDO/HMI data reached its maximum value. Analysis of the evolution of the microwave source that appeared before the X-flare in AR 12242 and comparison of its parameters with the parameters of other components of the AR microwave radiation showed that the new source can be classified as neutral line associated source (NLS), which were repeatedly detected by the RATAN-600 and other radio telescopes 1-3 days before the large flares.

Spectrum and physical conditions in microflare generation regions at decimeter-wave frequencies

The event of March 11, 2011 was used to study decimeter-wave microflares (MF) in solar active regions. A theoretical interpretation has been proposed for the nature and generation mechanism of decimeter-waveMFs, which is based on an analysis of the phenomenon of double plasma resonance and subsequent transformation of upper hybrid waves when they interact with low-frequency plasma waves. It is shown that MFs should form in the active regions between magnetic fields of opposite direction, where magnetic-field strength reaches 100–150 G in the transition region. We report the spectral properties of MFs computed with the allowance for inverse bremsstrahlung and cyclotron absorption and for the increment of upper-hybrid waves. It is shown that the transition region is the most likely place of MF generation within the framework of the model of electron-density and temperature. It is also shown that within the framework of electron density and temperature model in the active region the most likely MF generation place in the solar atmosphere is the transition region. MFs were observed at frequencies from 1.036 to 1.306 Hz, which is consistent with model computations.