Microturbulence at the front of supercritical quasiperpendicular shocks

In supercritical shocks a fraction of the incoming ions is reflected at the steep front and stream across the magnetic field in the foot. The drift of the reflected ion beam versus the electrons can easily destabilize waves in the electron cyclotron frequency range. By means of linear analysis, we show that several Bernstein harmonics can be unstable, their number being proportional to the drift, yet limited by the ion beam's temperature. By means of electromagnetic PIC simulations restricted to the foot, we investigate the nonlinear characteristics of these waves. First, high cyclotron harmonics develop in good agreement with linear dispersion properties. Second, as the high A;-modes saturate by trapping ions of the reflected beam, an inverse cascade occurs whereby the spectral power shifts toward lower A;-modes to eventually accumulate on the first harmonic. The late phase showcases the development of a magnetic component to the spectrum that had so far been mostly electrostatic. It also showcases a significant energy transfer from the ion beam to the electrons which experience a marked increase in temperature.