Vol 35, No 3 (2019)

The process of the rise and development of low-frequency instability of kinetic Alfven waves (KAW) and kinetic ion-acoustic waves (KIAW) in preflare solar plasma near the footpoints of magnetic loops, i.e., in the area that corresponds to the height of low-middle chromosphere, has been studied. The observational data obtained in the framework of international missions Hinode, SDO, and IRIS demonstrate that a magnetic field’s amplitude can vary in the interval from a few dozen up to few hundred gausses. The existence of large-scale weak electric fields (“sub-Dreicer” in generally accepted terminology) during a long enough time (with respect to the time of instability development) can be considered in the framework of the used concept as the main source of the wave generation. One more source of instability is slow drift motions of plasma, which are the result of spatial inhomogeneties of temperature and density of a medium. Our former investigations of the obtained solutions of dispersive relation for low-frequency kinetic waves, which are generated due to the development of correspond instabilities, have established an important fact: for some semiempirical models of the solar atmosphere, the kinetic waves generated during the linear stage of the development are members of the family of kinetic Alfven waves and the family of kinetic ion-acoustic waves. It has been proven that the considered wave generation can take place in plasma with pure Coulomb conductivity as well as in plasma with saturated, small-scale Bernstein turbulence. The latter can appear in the investigated area as result of natural evolution of instability of the first harmonics of quasi-Bernstein modes. This mode has much lower threshold with respect to the amplitude of a sub-Dreicer field than low-frequency kinetic waves. Besides this fact, the waves considered have a low enough degree of plasma nonisothermality, which is needed for the appearance of instability. The principal possibility for excitation of nondamped kinetic waves with small amplitudes in the area under investigation has also been proven. It is very important for the increase in the probability for realization of the process of three-wave interaction and appearance of the spikes of microwave emission in the preflare state of the active region and, correspondingly, for making the combined short-term prediction of a flare in it.

Microturbulence, macroturbulence, thermal motion, and rotation contribute to the broadening of line profiles in stellar spectra. Reliable data on the velocity distribution of turbulent motions in stellar atmospheres are needed to interpret the spectra of solar-type stars unambiguously in exoplanetary research. Stellar spectra with a high resolution of 115 000 obtained with the HARPS spectrograph provide an opportunity to examine turbulence velocities and their depth distributions in the photosphere of stars. Fourier analysis was performed for 17 iron lines in the spectra of 13 stars with an effective temperature of 4900–6200 K and a logarithm of surface gravity of 3.9–5.0 as well as in the spectrum of the Sun as a star. Models of stellar atmospheres were taken from the MARCS database. The standard concept of isotropic Gaussian microturbulence was assumed in this study. A satisfactory fit between the synthesized profiles of spectral lines and observational data verified the reliability of the Fourier method. The most likely estimates of turbulence velocities, the rotation velocity, and the iron abundance and their photospheric depth distribution profiles were obtained as a result. Microturbulence does not vary to any significant degree with depth, while macroturbulence has a marked depth dependence. The macroturbulence velocity increases with depth in the stellar atmosphere. The higher the effective temperature of a star and the stronger the surface gravity, the steeper the expected macroturbulence gradient. The mean macroturbulence velocity increases for stars with higher temperatures, weaker gravity, and faster rotation. The mean macro- and microturbulence velocities are correlated with each other and with the rotation velocity in the examined stars. The ratio between the macroturbulence velocity and the rotation velocity in solar-type stars varies from 1 (the hottest stars) to 1.7 (the coolest stars). The age dependence of the rotation velocity is more pronounced than that of the velocity of macroturbulent motions.