Statistical Analysis of High-frequency Whistler Waves at Earth's Bow Shock: Further Support for Stochastic Shock Drift Acceleration

We statistically investigate high-frequency whistler waves (with frequencies higher than ∼ 10 bow shock using Magnetospheric Multi-Scale (MMS) spacecraft observations. We focus specifically on the wave power within the shock transition layer, where we expect electron acceleration via stochastic sh ock drift acceleration (SSDA) to occur associated with efficient pitch-angle scattering by whistler waves. We find that the wave power is positively correlated with both the Alfvén Mach number in the normal incidence frame M_ A and in the de Hoffmann-Teller frame M_ A/cosθ_Bn. The empirical relation with M_ A/cosθ_Bn is compared with the theory of SSDA that predicts a threshold wave power proportional to (M_ A/cosθ_Bn)^-2. The result suggests that the wave power exceeds the theoretical threshold for M_ A / cosθ_Bn≳ 30-60, beyond which efficient electron acceleration is expected. This aligns very well with previous statistical analysis of electron acceleration at Earth's bow shock (M. Oka, G eophys. Res. Lett., 33, 5, 2006). Therefore, we consider that this study provides further support for SSDA as the mechanism of electron acceleration at Earth's bow shock. At higher-Mach-number astrophysical shocks, SSDA will be able to inject electrons into the diffusive shock acceleration process for subsequent acceleration to cosmic-ray energies.