Wave Emissions from the Lower-hybrid to the Electron Cyclotron Frequency in Quasi-perpendicular Shocks: a large-scale 2D PIC Simulation

In collisionless plasmas, supercritical shocks are characterized by a sizable fraction 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. We investigate the evolution of the unstable waves with a parallelized, full particle, 2D PIC code. The simulation box, made of 2048X2048 grid cells, can resolve narrow bands, short-wavelength Bernstein waves as well as oblique whistler waves with wavelengths as large as the ion inertia length. The spectral analysis reveals 4 types of wave emissions: (i) extended Bernstein waves with wavevectors pointing in a narrow cone about 90° to B_o, (ii) whistler waves with wavevectors kc/ω_pe 1 whose propagation is quasi-perpendicular, (iii) strongly oblique whistler waves with longer wavelengths. These 3 types 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]. The 4th type (iv) is not in Cerenkov resonance with either ion populations. It consists of EM waves with wavenumbers kc/ω_pe ≤ 1 whose propagation is quasi-parallel to B_o. Their complete characteristics (polarization, frequency, and phase speed) will be presented. We will make a comparison with MMS data [Hull and Muschietti, Cospar 2018] which, besides the strongly oblique whistlers with frequencies close to the lower-hybrid, also observes higher frequency whistlers in quasi-parallel propagation.