On the origin of the crescent-shaped distributions observed by MMS at the magnetopause

TL;DR

This paper investigates the origin of crescent-shaped electron velocity distributions at Earth's magnetopause, revealing they result from meandering orbits caused by magnetic field gradients, not ExB drifts, and predicts their occurrence in the magnetotail.

Contribution

The study demonstrates that crescent-shaped distributions are due to magnetic gradient-driven meandering orbits, challenging previous assumptions about ExB drift causality.

Findings

Crescent shapes are caused by meandering orbits, not ExB drifts.

High-resolution simulations support the orbit-based formation mechanism.

Prediction of crescent distributions in the magnetotail for future MMS observations.

Abstract

MMS observations recently confirmed that crescent-shaped electron velocity distributions in the plane perpendicular to the magnetic field occur in the electron diffusion region near reconnection sites at Earth's magnetopause. In this paper, we re-examine the origin of the crescent-shaped distributions in the light of our new finding that ions and electrons are drifting in opposite directions when displayed in magnetopause boundary-normal coordinates. Therefore, ExB drifts cannot cause the crescent shapes. We performed a high-resolution multi-scale simulation capturing sub-electron skin depth scales. The results suggest that the crescent-shaped distributions are caused by meandering orbits without necessarily requiring any additional processes found at the magnetopause such as the highly asymmetric magnetopause ambipolar electric field. We use an adiabatic Hamiltonian model of particle motion to confirm that conservation of canonical momentum in the presence of magnetic field gradients causes the formation of crescent shapes without invoking asymmetries or the presence of an ExB drift. An important consequence of this finding is that we expect crescent-shaped distributions also to be observed in the magnetotail, a prediction that MMS will soon be able to test.