Preferential Ion Heating and Particle Acceleration Downstream of Dispersive Shock Waves in Collisionless Multi-Ion Plasma
Abstract
We briefly review the theory of dispersive shock waves in collisionless multi-ion plasma. In such plasma, two (or more) fast magnetosonic wave modes exist: the high-frequency fast mode that propagates in the ion component with the higher thermal speed and the low-frequency fast mode that propagates in the ion component with the lower thermal speed [Toida and Aota, 2013; Zieger et al., 2015]. Both fast modes are dispersive on fluid and ion scales, which results in nonlinear dispersive shock waves. A negative dispersive wave mode produces a trailing wave train downstream of the shock, while a positive dispersive wave mode produces a precursor wave train upstream of the shock [Biskamp, 1973; Hoefer, 2014]. Here we present high-resolution three-fluid simulations of dispersive shock waves in two-ion-species plasma. We show that downstream propagating nonlinear magnetosonic waves grow until they steepen into shocklets (thin current sheets), overturn, and start to propagate backward in the frame of the downstream propagating wave, as predicted by theory [McKenzie et al., 1993; Dubinin et al, 2006]. The counter-propagating nonlinear waves result in fast magnetosonic turbulence far downstream of the shock. Interestingly, energy is transferred from small scales to large scales (inverse energy cascade) in the high-frequency fast mode, and from large scales to small scales (direct energy cascade) in the low-frequency fast mode as the turbulence develops in time. We show that the ion species with the lower thermal speed is preferentially heated by the turbulence. Forward shocklets can efficiently accelerate both ions and electrons to high energies through the shock drift acceleration mechanism. We can conclude that fast magnetosonic turbulence in collisionless multi-ion plasma will move the plasma towards a state where the thermal speeds of different ion species are comparable. Our theoretical and numerical simulation results could help to explain the observed preferential heating of heavy ions in the solar corona, the acceleration of energetic particles downstream of interpanetary shocks in the multi-ion solar wind, the non-adiabatic cooling of solar wind ions and pickup ions in the outer heliosphere, and the unfolding of the anomalous cosmic ray energy spectra in the heliosheath, downstream of the termination shock.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMSH23B3396Z
- Keywords:
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- 7829 Kinetic waves and instabilities;
- SPACE PLASMA PHYSICS;
- 7845 Particle acceleration;
- SPACE PLASMA PHYSICS;
- 7846 Plasma energization;
- SPACE PLASMA PHYSICS;
- 7851 Shock waves;
- SPACE PLASMA PHYSICS