Resonant micro-instabilities at quasi-parallel collisionless shocks: cause or consequence of shock (re)formation

Abstract

A case of two interpenetrating, cold and quasi-neutral ion-electron plasmas is investigated with the multi-fluid approach. We consider that one plasma flows quasi-parallel to the lines of a background magnetic field embedded in another static plasma. If the flow turns super-Alfvénic, we show that parallel R/L-modes and perpendicular X/O-modes become unstable and grow in amplitude. Within the linear theory, we find that the growth rate curve of an unstable mode has a maximum at some wavenumber specific to each mode. If we consider a shock-like plasma configuration, we find that the fastest growing mode is the resonant one (with $k\sim r_{gi}^{-1}$) which strongly interacts with ions. In Particle-In-Cell (PIC) simulations, we observe that a resonant wave with the same properties is excited during the early phases of shock formation. Once the wave becomes non-linear, it efficiently scatters ions and triggers the initial shock formation. This implies that the typical compression ratio of $\sim 4$ could naturally arise as a consequence of a highly resonant micro-physical process. We model the interaction of ions reflected from the reforming shock barrier in a weak-beam case, and we show that the upstream wave now matches the instability we expect from the equations. By using PIC simulations, we explain how the strong-beam resonant instability triggers shock formation in the non-linear stage, and how the weak-beam instability reforms and transmits the shock afterwards. The micro-instabilities that we study here could largely contribute to shock triggering as well as to the reformation and transmission of the shock itself.

arXiv commentThe following article has been accepted by Physics of Plasmas. After it is published, it will be found at https://publishing.aip.org/resources/librarians/products/journals/

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Cited by in corpus (6)

• PIC Simulation Methods for Cosmic Radiation and Plasma Instabilities
• Non-linear diffusive shock acceleration: A recipe for injection of electrons
• Electron acceleration at supernova remnants
• Particle acceleration in interstellar shocks
• On the distribution function of suprathermal particles at collisionless shocks
• Non-linear Diffusive Shock Acceleration of Cosmic Rays -- Quasi-thermal and Non-thermal Particle Distributions