Microturbulence within the foot of supercritical quasiperpendicular shocks: parametric analysis in reflected ions-electrons drifts
Résumé
Supercritical shocks are characterized by a fraction of the incoming ions which is reflected at the steep front and stream across the magnetic field in the foot. These ions accumulate and are responsible for the shock front self-reformation. 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. Separate full electromagnetic PIC periodic simulations restricted to the three local populations (incoming electrons/ions and reflected ions) identified in the foot region have been performed in order to investigate the nonlinear characteristics of these waves with a high spatial resolution and a high statistics. First, high cyclotron harmonics develop in good agreement with linear dispersion properties and over times much smaller that the characteristic period of the shock front self-reformation. Second, as the high k-modes saturate by trapping ions of the reflected beam, the spectral power shifts towards lower k-modes to eventually accumulate on the first harmonic in a process which appears like an inverse cascade. Third, one surprising result in the late phase is the development of a magnetic component to the spectrum that had so far been mostly electrostatic. Fourth, the late phase also evidences a significant energy transfer from the ion beam to the electrons which experience a marked increase in temperature.