Microturbulence within the Front of a Quasi-Perpendicular Supercritical Shock: Comparison between Theory, Simulation and Space Observations

Abstract:

Supercritical shocks are characterized by a noticeable fraction of the incoming ions which is reflected at the steep front, stream across the magnetic field and form a foot upstream of the ramp. These ions accumulate and have several impacts: (i) these are responsible for the shock front self-reformation, (ii) these carry a significant amount of energy and play a key role in transforming the directed bulk energy (upstream) into thermal energy (downstream) and (iii) are source of microturbulence within the shock front itself. Indeed, the relative drift between the reflected ion beam and the incoming electrons within the foot can easily destabilize waves (electron cyclotron drift instability or ECDI) in the electron cyclotron frequency range. By means of linear analysis, several Bernstein harmonics are shown to be unstable, their number being proportional to the drift, yet limited by the ion beam’s temperature. Separate electromagnetic PIC simulations restricted to all ions and electrons populations of 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 time evolves, the high k-modes saturate and the spectral power shifts toward lower k-modes to eventually accumulate on the first harmonic; this unexpected feature is shown as being due to the contribution of both processes: (i) the ion trapping and (ii) the resonance broadening. Third, one surprising result in the late nonlinear phase is the development of a magnetic component to the spectrum that had so far been mostly electrostatic. Fourth, present simulations also evidence a significant energy transfer from the ion beam to the electrons which experience a marked increase in temperature: a preheating takes place before electrons reach the ramp which may impact the global energy partition through the shock front. Results will be compared with experimental space data.