On the origin of short large‐amplitude magnetic structures upstream of quasi‐parallel collisionless shocks

Abstract

In order to model the development of the turbulence upstream of quasi‐parallel shocks, one‐dimensional hybrid simulations have been performed of the injection of a very hot ion population into a cold incident flow. ULF electromagnetic waves are initially generated in the right‐hand resonant ion beam mode. As these waves are convected to the injection source and encounter larger beam densities, they grow in amplitude and scatter the beam ions. Beam ion clumps characterized by distributions with a significant fraction of ions having velocities antiparallel to the beam bulk velocity are generated, and destabilize the left‐hand resonant ion beam mode. This process enables the formation of enhanced magnetic field structures which, like the short large‐amplitude magnetic structures (SLAMS) observed upstream of the quasi‐parallel Earth's bow shock, show scale lengths smaller than the ULF waves wavelength, have magnetic amplitudes up to a few times the ambient magnetic field, are left‐hand polarized in the plasma rest frame, and often possess attached whistler waves.