Generation of Low-Frequency Kinetic Waves at the Footpoints of Pre-Flare Coronal Loops

Alexandr Kryshtal,Viktor Fedun,Gary Verth,Oleg Cheremnykh,Istvan Ballai,Anna Voitsekhovska

doi:10.1007/s11207-020-01725-w

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 to be in a pre-flare state. It is shown that among the well-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 Alfvén 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 presence of spatial inhomogeneities. We also show that the generation of kinetic Alfvén 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 the development of low threshold instability in plasma with relatively low values of the magnetic field strength is also discussed.

Highlights

High-resolution solar observational data obtained during Hinode, SDO, STEREO, IRIS missions have confirmed that the coronal heating problem will most likely only be solved if the photosphere, chromosphere, transition region and corona are considered together as a connected energetic circuit
Among the great number of waves that can appear in a magneto-active plasma (Alexandrov, Bogdankevich, and Rukhadze, 1984), two in particular are of great interest, i.e. the kinetic Alfvén wave (KAW) and the kinetic ion-acoustic wave (KIAW)
The problem of the influence of small-scale Bernstein turbulence on low-frequency waves is a very important aspect of our problem and it has been previously investigated by Kryshtal et al (2017). Their analysis showed that second harmonics of oblique quasiBernstein modes are possible in the range of magnetic field that is characteristic in pre-flare active regions (ARs) chromospheres

Summary

Introduction

High-resolution solar observational data obtained during Hinode, SDO, STEREO, IRIS missions have confirmed that the coronal heating problem will most likely only be solved if the photosphere, chromosphere, transition region and corona are considered together as a connected energetic circuit (see, e.g., Aschwanden, 2001, 2005). Among the great number of waves that can appear in a magneto-active plasma (Alexandrov, Bogdankevich, and Rukhadze, 1984), two in particular are of great interest, i.e. the kinetic Alfvén wave (KAW) and the kinetic ion-acoustic wave (KIAW) Both types of waves have unique properties (see, e.g., Hasegawa and Chen, 1976a) and can effectively accelerate particles, (Miller et al, 1997), take part in processes of three-wave interaction (see, e.g., Hasegawa and Chen, 1976b; Brodin, Stenflo, and Shukla, 2006), support the development of turbulence (which is a necessary element in the process of formation of pre-flare current sheets) and can cause abnormal resistivity (Somov, 1994). Our study can be applied to the situation when flares occur in succession in the same AR, i.e. when the percentage of energetic particles after the first flare has become too small to influence the onset of the succeeding flare (see, e.g., Harra, Matthews, and Culhane, 2001)

Pre-Flare Plasma Model and Dispersion Relation

Investigation of Kinetic and EM Wave Generation and Stability

Results and Discussion