Crescent‐shaped electron distributions at the non‐reconnecting magnetopause: Magnetospheric Multiscale observations

B. ‐B. Tang  W. Y. Li  D. B. Graham  A. C. Rager  C. Wang  Yu. V. Khotyaintsev  B. Lavraud  H. Hasegawa  Y. ‐C. Zhang  L. Dai  B. L. Giles  J. C. Dorelli  C. T. Russell  P. ‐A. Lindqvist  R. E. Ergun J. L. Burch
First published: 01 March 2019 https://doi.org/10.1029/2019GL082231

Crescent‐shaped electron distributions perpendicular to the magnetic field are an important indicator of the electron diffusion region in magnetic reconnection. They can be formed by the electron finite gyroradius effect at plasma boundaries or by demagnetized electron motion. In this study, we present Magnetospheric Multiscale (MMS) mission observations of electron crescents at the flank magnetopause on 2017 September 20, where reconnection signatures are not observed. These agyrotropic electron distributions are generated by electron gyromotion at the thin electron‐scale magnetic boundaries of a magnetic minimum after magnetic curvature scattering. The variation of their angular range in the perpendicular plane is in good agreement with predictions. Upper hybrid waves are observed to accompany the electron crescents at all four MMS spacecraft as a result of the beam‐plasma instability associated with these agyrotropic electron distributions. This study suggests electron crescents can be more frequently formed at the magnetopause.