Flow-Regulated Suprathermal Particle Acceleration in Weakly Collisional Astrophysical Plasmas

Abstract

We investigate the formation of suprathermal particle populations in weakly collisional plasmas using a one-dimensional Fokker-Planck framework. A key element of this work is the introduction of a systematic velocity-space drift term that represents net energization relative to a background streaming flow. This term provides a minimal phenomenological description of competing relaxation and acceleration processes, enabling the incorporation of large-scale plasma dynamics into kinetic particle evolution. The model further includes spatial advection, velocity-space diffusion associated with wave-particle interactions, and collisional relaxation. To explore the role of time-dependent large-scale plasma dynamics, we consider three representative temporal profiles of the streaming velocity: accelerating, decelerating, and steady flows. We find that velocity-space diffusion primarily governs the formation of suprathermal tails, while the streaming-induced drift regulates their efficiency and spectral properties. In particular, the overall fraction of suprathermal particles depends only weakly on the detailed temporal evolution of the flow and is largely controlled by the time-averaged streaming velocity. These results suggest that large-scale streaming motions can be incorporated as an effective systematic energization mechanism in weakly collisional plasmas, providing a minimal and flexible framework applicable to a broad range of space and astrophysical environments.