  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.
