Acceleration of Thermal Protons By Generic Phenomenological Mechanisms

TL;DR

This paper models proton heating and acceleration in hot astrophysical plasmas, revealing the conditions under which nonthermal tails form and their implications for X-ray emissions, while highlighting the constraints on nonthermal processes due to plasma heating.

Contribution

It introduces a coupled kinetic model for protons and electrons, solving Fokker-Planck equations to analyze proton acceleration and plasma heating in astrophysical environments.

Findings

Nonthermal proton tails can form but cause excessive plasma heating.

Steeper turbulence spectra than Kolmogorov are needed to avoid overheating.

Nonthermal tails may serve as seeds for further acceleration.

Abstract

We investigate heating and acceleration of protons from a thermal gas with a generic diffusion and acceleration model, and subject to Coulomb scattering and energy loss, as was carried out in Petrosian & East (2008) for electrons. As protons gain energy their loss to electrons becomes important. Thus, we need to solve the coupled proton-electron kinetic equation. We numerically solve the coupled Fokker-Plank equations and computes the time evolution of the spectra of both particles. We show that this can lead to a quasi-thermal component plus a high energy nonthermal tail. We determine the evolution of nonthermal tail and the quasi-thermal component. The results may be used to explore the possibility of inverse bremsstrahlung radiation as a source of hard X-ray emissions from hot sources such as solar flares, accretion disk coronas and the intracluster medium of galaxy clusters. We find that emergence of nonthermal protons is accompanied by excessive heating of the entire plasma, unless the turbulence needed for scattering and acceleration is steeper than Kolmogorov and the acceleration parameters, the duration of the acceleration, and/or the initial distributions are significantly fine-tuned. These results severely constraint the feasibility of nonthermal inverse bremsstrahlung process producing hard X-ray emissions. However the nonthermal tail may be the seed particles for further re-acceleration to relativistic energies, say by a shock. In the Appendix we present some tests of the integrity of the algorithm used and present a new formula for the energy loss rate due to inelastic proton-proton interactions.