Multi-channel coupling of decay instability in three-dimensional low-beta plasma

Horia Comişel,Yasuhito Narita,Uwe Motschmann

doi:10.5194/angeo-37-835-2019

Abstract

Abstract. Three-dimensional hybrid simulations have been carried out to verify the hypothesis of simultaneous multi-channel decay of a large-amplitude Alfvén wave in a low-beta plasma, e.g., in the shock-upstream region or the solar corona. Obliquely propagating daughter modes are excited along the perpendicular direction to the mean magnetic field at the same parallel wavenumbers and frequencies as the daughter modes driven by the field-aligned decay. We find that the transversal spectrum of waves is controlled by the multi-channel coupling of the decay process in low-beta plasmas and originates in the dispersion state of the shear Alfvén wave.

Highlights

Quasi-parallel-propagating Alfvén waves with circular polarization are frequently observed in various space plasma domains, in particular in the vicinity of the Earth’s bow shock or other planetary bow shocks, where the shock-reflected ions transfer energy from the ion beam into electromagnetic waves through the ion beam instability
The reason for the multichannel coupling lies in the fact that the decay instability in three-dimensional low-beta plasmas occurs simultaneously in various directions imposed by the mean magnetic field and has a degeneration over the directions perpendicular to the mean field
Earlier studies on the nonlinear interaction between Alfvén waves and obliquely propagating waves report that the growth rates of the decay instability in the oblique direction to the mean magnetic field are typically smaller than the case of field-aligned parametric instabilities

Summary

Introduction

Quasi-parallel-propagating Alfvén waves with circular polarization are frequently observed in various space plasma domains, in particular in the vicinity of the Earth’s bow shock or other planetary bow shocks (see, e.g., Narita et al, 2007), where the shock-reflected ions transfer energy from the ion beam into electromagnetic waves through the ion beam instability (often right-hand resonance in which the gyration of the beam ions is observed as right-hand polarized in the rest frame of the core or thermal plasma). Earlier studies on the nonlinear interaction between Alfvén waves and obliquely propagating waves (see, e.g., Mjølhus and Hada, 1990; Viñas and Goldstein, 1991a, b; Laveder et al, 2002; Nariyuki et al, 2008) report that the growth rates of the decay instability in the oblique direction to the mean magnetic field are typically smaller than the case of field-aligned parametric instabilities Numerical simulations such as multi-dimensional MHD (magnetohydrodynamics) and hybrid plasma simulations lead to the conclusion with respect to the circular polarization that the parametric decay of large-amplitude Alfvén waves develops along the propagation direction of the pump wave (see, e.g., Del Zanna et al, 2001; Verscharen et al, 2012; Gao et al, 2013). Our goal is to study the hypothesis or the scenario of the multi-channel coupling by running a threedimensional hybrid plasma simulation in a low-beta plasma setup

Simulation run

Growth rate estimate

Discussion and conclusions