Agyrotropic Electron Distributions in the Foreshock Region

Agyrotropic electron distributions are usually taken as an important indicator of the electron diffusion region in magnetic reconnection, while they are also found at electron-scale boundaries of the non-reconnecting magnetopause. In this study, we present Magnetospheric Multiscale (MMS) observations of agyrotropic electron distributions in the foreshock region, where reconnection signatures are absent. These agyrotropic electron distributions are observed in a hot flow anomaly (HFA) and short large-amplitude magnetic structure (SLAMS), respectively, which are generated by the electron finite gyroradius effect at the thin electron-scale magnetic boundaries after magnetic curvature scattering. We reproduce the agyrotropic electron distributions by using test particle method under the assumption of a simple 1D magnetic geometry. This study indicates that agyrotropic electron distributions can be more frequently formed at electron-scale boundaries in space plasma.