On the Nonlinear Interaction of Low-Frequency Kinetic Plasma Waves in the Preflare Atmosphere of the Solar Active Region

Necessary conditions for the appearance of decay instability of low-frequency kinetic Alfven waves in loop structures of an active region on the Sun before a flare are obtained. On the basis of the transcendental dispersion equation obtained using the synchronism conditions and additional conservation laws, an expression is obtained for the nonlinear growth rate of the decay instability of the initial kinetic Alfven wave (KAW) to the secondary KAW and the kinetic ion-acoustic wave (KIAW). Boundary values of the reduced amplitude of the initial KAW in the preflare atmosphere of an active region (AR) are obtained. It was assumed in the calculations that the waves involved in the process in the AR under study appear due to the development of instability caused by the presence of a weak large-scale (sub-Dreicer) electric field and drift motions of the plasma due to spatial inhomogeneities of its temperature and density. It is shown that, for a certain type of semiempirical model of the solar atmosphere, kinetic Alfven and kinetic ion-acoustic waves can be generated during the linear stage of instability development. In this case, the process of wave generation can take place both in plasma with purely Coulomb conductivity and in the presence of small-scale Bernstein turbulence in it. To “trigger” the instability, relatively low values of plasma nonisothermality and a very low threshold value of the reduced amplitude of the sub-Dreicer electric field are required. The possibility of the appearance of undamped kinetic waves of small amplitude in the region under study is proven.