Electron Scattering by Low-Frequency Whistler Waves at Earth's Bow Shock

Electrons are accelerated to non-thermal energies at shocks in space and astrophysical environments. While Shock Drift Acceleration (SDA) has been considered a key process of electron acceleration at Earth's bow shock, it has also been recognized that SDA needs to be combined with an additional stochastic process to explain the observed power-law energy spectra. Here we show mildly energetic (~0.5 keV) electrons are locally scattered (and accelerated while being confined) by magnetosonic-whistler waves within the shock transition layer especially when the shock angle is large (θ _Bn >~ 70°). When measured by the Magnetospheric MultiScale (MMS) mission at a high cadence, ~0.5 keV electron flux increased exponentially in the shock transition layer. However, the flux profile was not entirely smooth and the fluctuation showed temporal/spectral association with large-amplitude (δB/B ~ 0.3), low-frequency (<~0.1 Ω _ce where Ω _ce is the cyclotron frequency), obliquely-propagating (θ _Bk ~ 30° - 60° where θ _Bk is the angle between the wave vector and background magnetic field) whistler waves, indicating that the particles were interacting with the waves. Particle simulations demonstrate that the observed electron-whistler interaction is intrinsically non-linear and electrons can be scattered beyond 90° pitch angle.