Low-Frequency Kinetic Waves in Plasmas of Magnetic Loops at the Early Stage of a Flare Process in an Active Region

Abstract

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.