Generation of Suprathermal Electrons by Collective Processes in Collisional Plasma

TL;DR

This paper proposes a new physical mechanism involving electrostatic bremsstrahlung emission that explains the formation of suprathermal electron tails in collisional plasmas, aligning with observations in space environments.

Contribution

It introduces electrostatic bremsstrahlung as a fundamental process generating suprathermal electrons in highly collisional plasmas, supported by numerical solutions of kinetic equations.

Findings

Steady-state electron distribution has Maxwellian core plus power-law tail.

Electrostatic bremsstrahlung modifies Langmuir wave spectra.

Initial Maxwellian distribution evolves into a suprathermal tail.

Abstract

The ubiquity of high-energy tails in the charged particle velocity distribution functions observed in space plasmas suggests the existence of an underlying process responsible for taking a fraction of the charged particle population out of thermal equilibrium and redistributing it to suprathermal velocity and energy ranges. The present Letter focuses on a new and fundamental physical explanation for the origin of suprathermal electron distribution function in a highly collisional plasma. This process involves a newly discovered electrostatic bremsstrahlung emission that is effective in a plasma in which binary collisions are present. The steady-state electron velocity distribution function dictated by such a process corresponds to a Maxwellian core plus a quasi-inverse power-law tail, which is a feature commonly observed in many space plasma environment. In order to demonstrate this, the system of self-consistent particle- and wave- kinetic equations are numerically solved with an initially Maxwellian electron velocity distribution and Langmuir wave spectral intensity, which is a state that does not reflect the presence of electrostatic bremsstrahlung process, and hence not in force balance. The electrostatic bremsstrahlung term subsequently drives the system to a new force-balanced steady state. After a long integration period it is demonstrated the initial Langmuir fluctuation spectrum is modified, which in turn distorts the initial Maxwellian electron distribution into a velocity distribution that resembles the said core-suprathermal velocity distribution. Such a mechanism may thus be operative at the coronal source region, which is characterized by high collisionality.