Wave excitation and turbulence at the Earth's bow shock: cluster data and simulations

Abstract

Wave-particle interactions are the major player in shock formation, particle reflection and acceleration, downstream thermalization, and wave excitation. In particular, the foreshock is characterized by extensive wave activity, which is caused by the interaction of the backstreaming particles with the inflowing solar wind. This interaction causes instabilities, which in turn excite ultra-low-frequency (ULF) magnetohydrodynamic waves. ULF waves generated upstream of the quasi-parallel shock have been shown, in numerical simulations to be convected back to the shock front, where they contribute to its destabilization and re-formation. In the quasi-perpendicular regime, beams of reflected ions create waves that are convected downstream, mediating the shock structure and/or the quasi-parallel regime, depending on how far upstream they are created. These waves, and the waves produced in the shock layer, are responsible for downstream thermalization and they are an important source of waves and turbulence downstream of the shock. In this paper we will review select Cluster observations at the Earth's bow shock and recent kinetic simulations in this context. In particular, we will review particle reflection, downstream thermalization, and wave excitation in the foreshock region.