On the formation of quasi-parallel shocks, magnetic and electric field turbulence, and the ion energization mechanism

Abstract

ABSTRACT The analysis of four-point measurements by the NASA Magnetospheric Multiscalespacecraft reveals that the formation of quasi-parallel shocks is consistent with the counter-streaming ion–ion resonant right-handed instability. The counter-streaming occurs between the incoming solar wind and a diffuse hot ion population moving upstream. It is found that the onset of the instability is initiated by 30 s period oscillations, which steepen to shorter wavelengths and higher frequencies. The turbulent cascade in the electric field ranges from 0.01 Hz to a few kHz. The dominant frequency of the instability in the magnetic field is observed around the proton cyclotron frequency, consistent with the theoretical predictions. The instability produces large amplitude magnetic field and plasma density shocklets with the perpendicular scale length of 300 km (3 proton gyroradii) and the parallel scale length three times larger. The active acceleration regions are characterized by non-gyrotropic ion populations in the velocity space, in addition to the solar wind beam and a diffuse hot beam propagating upstream. We show that non-gyrotropic ion populations are consistent with the $ExB$ wave energization mechanism that can both heat bulk plasma and accelerate some ions to the observed energies of 200 keV.