Generation of low-frequency kinetic waves at the footpoints of pre-flare coronal loops

Abstract

In this study we discuss the excitation of low frequency plasma waves in the lower-middle chromosphere region of loop footpoints for the case when the plasma can be considered in a pre-flare state. It is shown, that among the known semi-empirical models of the solar atmosphere, only the VAL (F) model together with a particular set of basic plasma parameters and amplitudes of the electric and magnetic fields supports generation of low frequency wave instability. Our results show that it is possible to predict the onset of the flare process in the active region by using the interaction of kinetic Alfven and kinetic ion-acoustic waves, which are solutions of the derived dispersion equation. The VAL (F) model allows situations when the main source of the aforementioned instability can be a sub-Dreicer electric field and drift plasma movements due to the presence of spatial inhomogeneities. We also show that the generation of kinetic Alfven and kinetic ion-acoustic waves can occur both, in plasma with a purely Coulomb conductivity and in the presence of small-scale Bernstein turbulence. The excitation of the small amplitude kinetic waves due to development of low threshold instability in plasma with relatively low values of the magnetic field strength is also discussed.