Generation of Low-Frequency Kinetic Waves at the Footpoints of Pre-Flare Coronal Loops
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Generation of Low-Frequency Kinetic Waves at the Footpoints of Pre-Flare Coronal Loops Alexandr Kryshtal1 · Anna Voitsekhovska1 · Oleg Cheremnykh1 · Istvan Ballai2 · Gary Verth2 · Viktor Fedun3
Received: 11 May 2020 / Accepted: 20 October 2020 / Published online: 24 November 2020 © The Author(s) 2020
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. Keywords Plasma instabilities · Solar atmosphere · Flares
B V. Fedun
[email protected] A. Kryshtal [email protected] I. Ballai [email protected] G. Verth [email protected]
1
Space Research Institute, Kiev, 03680, 187, Ukraine
2
Plasma Dynamics Group, School of Mathematics and Statistics, The University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK
3
Plasma Dynamics Group, Department of Automatic Control and Systems Engineering, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK
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1. 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). In this regard present theoretical models that attempt to explain the problem of coronal heating have a number of limitations as regards their applicability. These limitations are related to the spatial inhomogeneity of the plasma (see, e.g., Aschwanden, 2001, 2005; Heinzel, Dorotoviˇc, and Rutten, 2007; Reale, 2010), the location of the regions of the primary energy release of some flares in active regions (ARs) (Reznikova et al., 2009; Zaitsev, Stepanov, and Tsap, 1994; Charikov and Shabalin, 2015), anomalously
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