Impact of the nonstationarity of a supercritical perpendicular collisionless shock on the dynamics and energy spectra of pickup ions

Abstract

[1] Both hybrid and full particle simulations and recent experimental results have clearly evidenced that the front of a supercritical quasi-perpendicular shock can be nonstationary. One proposed mechanism responsible for this nonstationarity is the self-reformation of the shock front being due to the accumulation of reflected ions. On the other hand, a large number of studies have been made on the acceleration and heating of pickup ions (PIs) but most have been restricted to a stationary shock profile only. Herein, one-dimensional test particle simulations based on shock profiles issued from one-dimensional particle-in-cell simulation are performed in order to investigate the impact of the shock front nonstationarity (self-reformation) on the acceleration processes and the resulting energy spectra of PIs (protons H+) at a strictly perpendicular shock. PIs are represented by different shell distributions (variation of the shell velocity radius). The contribution of shock drift acceleration (SDA), shock surfing acceleration (SSA), and directly transmitted (DT) PI's components to the total energy spectra is analyzed. Present results show that (1) both SDA and SSA mechanisms can apply as preacceleration mechanisms for PIs, but their relative energization efficiency strongly differs; (2) SDA and SSA always work together at nonstationary shocks (equivalent to time-varying shock profiles) but SDA, and not SSA, is shown to dominate the formation of high-energy PIs in most cases; (3) the front nonstationarity reinforces the formation of SDA and SSA PIs in the sense that it increases both their maximum energy and their relative density, independently on the radius of PI's shell velocity; and (4) for high shell velocity around the shock velocity, the middle energy range of the total energy spectrum follows a power law Ek−1.5. This power law is supported by both SDA and DT ions (within two separate contributing energy ranges) for a stationary shock and mainly by SDA ions for a nonstationary shock. In both cases, the contribution of SSA ions is comparatively weak.