Whistler Waves and Two‐Stream Instabilities at Non‐Stationary Quasi‐Perpendicular Collisionless Shocks

Abstract Upstream propagating waves are observed to correlate with the reflected ions in the foot of a high Mach number quasi‐perpendicular collisionless shock. The respective wavefronts show time varying amplitude matching the variation of the reflected ions and the shock reformation cycles. We interpret this process in terms of the fast two‐stream instability and investigate the results of the PIC simulation as compared with the prediction of the linear analysis. Evidence for upstream propagating whistler waves supported by two‐stream instability is continuously growing, see for example, recent surveys based on MMS in‐situ observations of the Earth bow‐shock. The present study suggests that upstream propagating waves are consistent with and in particular supported by the mechanism of fast two‐stream instability developing in the upstream region of the shock foot.