Impact of secondary electron instabilities in the wave emissions excited within the front of a quasi-perpendicular shock: 2D PIC simulation

Supercritical collisionless shocks are characterized by a sizeable percentage of ions that are reflected off of the shock front. These ions carry a significant amount of energy and are the source of microturbulence. They play a key role in transforming the directed bulk energy (upstream) into thermal energy (downstream). For quasi-perpendicular shock geometries, the reflected ions are mostly directed at 90° to the magnetic field B_o, which results in streaming instabilities that are excited by the relative drifts between incoming ions, reflected ions, and electrons across B_o. Previous recent 2D PIC simulations (Muschietti et al., AGU 2018; AOGS 2019) have investigated in details the competition and the mutual interactions between different « primary » , streaming instabilities : (i) Electron Bernstein instability with wavevectors close to perpendicular to B_o, (ii) and (iii) two types of modified two-stream instabilities in the lower-hybrid frequency range. These 3 instabilities are in Cerenkov resonance with either the incoming ion beam or the reflected ion beam, in agreement with our previous study (Muschietti and Lembege, Ann. Geophys., 2017). On the other hand, a 4th type (iv), « secondary » instability emerges (high-frequency whistler, or WHI) which is not in resonance with either ion populations yet appears as a by-product of the Bernstein instability (i). The features of this WHI are analyzed in details and are shown (a) to complete the frequency range of all emissions within the front of the shock, (b) to coexist with other primary instabilities and (c) to impact the global electromagnetic energy partition. Consequences for MMS experimental results will be also discussed.