Two-stream Instabilities within the Front of Supercritical Quasi-perpendicular Shocks: a Synthetic Analysis

Abstract:

In supercritical shocks a substantial fraction of ions is reflected at the steep shock ramp and carries a considerable amount of energy. The presence of reflected ions enables streaming instabilities to develop across the foot's magnetic field. These are excited by relative drifts between the populations of incoming ions, reflected ions, and electrons. The instabilities cover wavelengths from the ion inertia length to the electron gyroradius and frequencies from the lower-hybrid to the electron cyclotron. The particle distributions are modelled as three components: a broad electron and two ion populations, viz. a core and a beam representing the reflected ions. Assuming the ion beam is directed along the shock normal at 90^o to B_o, we investigate the possible instabilities under various wave propagation angles.

Three types of waves are shown to be unstable: 1) Oblique whistlers with wavelength about the ion inertia length which propagate toward upstream at angles about 50^o to B_o. Frequencies are a few times the lower-hybrid. The waves share many similarities to the oblique whistlers measured in detail by Polar [Hull et al., JGR 117, 2012]. 2) Quasi-perpendicular whistlers with wavelength covering a fraction of the electron inertia length which propagate toward downstream at angles larger than 80^o to B_o. Frequencies are close to the lower-hybrid. 3) Bernstein waves with wavelength close to the electron gyroradius which propagate toward upstream at angles within 5^o of perpendicular to B_o. Frequencies are close to the electron cyclotron. The waves have similarities to those reported by Wind and Stereo [Breneman et al., JGR 118, 2013; Wilson et al., JGR 115, 2010].

Linear dispersion properties are analyzed by computing the full electromagnetic dielectric tensor. Then, dispersion results are compared with waveforms and hodograms obtained from electromagnetic pseudo-oblique 1D PIC simulations. One computes the Poynting flux associated to the whistlers. It is directed toward upstream for the oblique whistlers, as expected. We present a synthetic view of wave emissions of two-stream origin and connect our results with the low-frequency whistlers of Hellinger and Mangeney [JGR 102, 1997], the MTSI-1 and 2 of Matsukyio and Scholer [JGR 111, 2006], and the Bernstein waves of Muschietti and Lembege [JGR 118, 2013].