On the nonstationarity of collisionless shocks and its impact on deriving the cross-shock potential

摘要

Recently, Comisel et al. (2015) suggested an adaptation of the deHoffmann-Teller (HT) frame, to take into account the magnetic field fluctuations observed nearby and inside a quasi-perpendicular collisionless shock. Such fluctuations can be quite large, making the standard HT frame unsuitable for studies of the electric field, cross-shock potential, and electron energy gain. While the HT frame is appropriate to cancel the motional electric field for steady state shocks, a similar effect is achieved with the adaptive HT (AHT) frame under time varying conditions, albeit by a local rather than global transformation. The sliding AHT motion parallel to the shock surface changes from point to point and time to time, unlike the regular motion of the HT frame. On the other hand, in both cases the electric field at each point is normal to the 1-D shock and its integral across the shock is equal to the integral of the parallel electric field along the magnetic field line. In this paper we elaborate the concept of the AHT frame, illustrate it with a 1-D simulation, and point out the association of magnetic field fluctuations with electron phase space structures, apparently related to electron trapping by dispersive whistler waves in the foot of the shock. The AHT frame may help assessing the relative weight of nonstationarity at shocks and other MHD discontinuities, by simulated and observed data, under various plasma regimes.