Yu. V. Tikhomirov

Microwave emission from hot X-ray kernels in solar flares

Spectral and polarization characteristics of radio emission from hot X-ray kernels are considered. It is shown that the frequency spectrum of thermal cyclotron radio emission and bremsstrahlung from these regions may contain a set of lines at cyclotron harmonics and the maximum at plasma frequency. This makes possible the diagnostics of the X-ray kernel plasma and magnetic fields according to spectral and polarization observations of microwave radio emission in solar flares.

Manifestation of pulsation instability in solar radio emission preceding proton flares

The investigation of solar radio emission fluctuations at the wavelength λ ∼- 3 cm led to the discovery of a visible increase in pulsations with periods of about 30–120 min prior to proton flares. These pulsations were observed before all (seven) proton flares included in our cycle of observations from 1969 to 1974. The phenomenon was not found to occur before non-proton flares. The assumption is made that the observed pulsations are a manifestation of pre-flare instability in coronal structures. Estimations have been made for fluctuations of the gyro-resonance radiation from the regions above spots associated with the magnetic field variations when a groove instability of a coronal condensation is developed. They are in good agreement with the observational data. The discovered manifestation of the pre-flare instability in fluctuations of the solar radio emission open new ways to study the flare development and to predict geo-effective phenomena on the Sun.

On the amplitude of the radio response of the active region magnetosphere to long-period natural oscillations of sunspots

The following has been indicated for the cyclotron emission of microwave sources based on a simple modeling of a unipolar sunspot magnetosphere by the magnetic monopole, submerged in subphotospheric layers, and a temperature, changing with altitude as hyperbolic tangent. At insignificant changes in the sunspot magnetic field strength, originating as a result of oscillations of a sunspot as an integrated structure, the oscillation amplitude (in percent) of brightness temperatures of the Stokes I and V parameters at a wavelength of 1.76 cm (NoRH) is an order of magnitude as large as the magnetic field strength amplitude. This amplitude is of the same order of magnitude as the field oscillation amplitude at a wavelength of 5.2 cm (SSRT).

The Results of Studying Nonstationary Processes on the Sun by Radioastronomical Methods

This paper presents the results of studies of solar phenomena performed at the Radiophysical Research Institute (NIRFI) in the last years and based on the data of measuring the solar microwave radiation. We obtained new results concerning fragmentation of the regions of initial energy release during solar flares, their spatial and temporal dynamics, the physical conditions in flaring loops, the dynamics of energetic particles accelerated during flares, and the characteristics of sporadic radio emission during the formation and propagation of coronal mass ejections in the lower layers of the solar atmosphere.

VLBI studies of the solar-wind plasma irregularities at 18 and 92 CM wavelengths in 1994–1996

We discuss the capabilities of VLBI studies of irregularities of the solar-wind plasma using multi-element radio interferometric facilities. We analyze the data obtained during international VLBI experiments at decimeter wavelengths (92 and 18 cm) in 1994–1996 and supposed that the irregularities have a “stream” structure. The “streams” are strongly elongated in the solar-wind direction (longitudinal size of about a few hundred thousand kilometers) and have the transverse size of about 0.5–2.0 thousand kilometers. The irregularities inside a single “stream” are almost isotropic. We discuss the restrictions imposed on operation of decimeter VLBI systems due to effects of the interplanetary medium.

Radiointerferometric studies of the fine structure of supercompact extragalactic sources and space media in the decimeter wavelength range

We describe international VLBI experiments at decimeter wavelengths (92 and 18 cm) carried out in 1994–1997. These experiments were organized as a pre-lauch survey of space radio sources for the forthcoming Earth-space radio interferometer “Radioastron.” The experiments were aimed at determination of the list of the sources that can be used in future observations with the space-Earth interferometer, determination of the limits posed by the interplanetary medium on such observations, and study of the physical parameters of irregularities of the solar wind plasma. The first obtained results are reported.

Radio interferometric observations of solar bursts in the decimeter range with millisecond temporal resolution

We describe the radio interferometric equipment for observation of solar spike-like events at decimeter wavelengths with millisecond time resolution. The equipment was tested using a two-element interferometer (2 RT7, f=540 MHz, baseline 416 m) at the “Staraya Pustyn” radioastronomical station (RAS) of the Radiophysical Research Institute of Nizhny Novgorod (NIRFI) in October–November 1993 and in September–October 1994. Over 20 solar bursts of different durations, which showed considerable interference response, were detected with time resolution 0.256 ms. The typical time variation scale of the bursts was from 50 to 200 ms. Bursts with time scales less than 20 ms were absent. The actual possibility of measuring the displacements of radio source brightness centers with accuracy up to 1–2 arc s is shown.

Possibilities of VLBI observation of solar spike-like events

Modern theoretical and experimental investigations of energy release fragmentation in solar flares show the necessity of observations with high space and time resolution. In this paper, we analyze earlier interferometric observations of solar radio spikes and discuss the first results of the Radiophysical Research Institute of Nizhny Novgorod (NIRFI) in this field. We describe the NIRFIs receiving and analyzing equipment, which is part of the measuring complex for solarflare investigation with high space and time resolution and enables one to analyze the form of the interference signal and its amplitude spectrum with time and frequency resolutions of up to 16.7 μsec and 100 Hz, respectively. The prospects for VLBI observation of solar flares with high space resolution are discussed.