Electrostatic turbulence and Debye-scale structures in collisionless shocks
2020
We present analysis of more than one hundred large-amplitude bipolar
electrostatic structures in a quasi-perpendicular supercritical Earth's bow
shock crossing, measured by the Magnetospheric Multiscale spacecraft. The
occurrence of the bipolar structures is shown to be tightly correlated with
magnetic field gradients in the shock transition region. The bipolar structures
have negative electrostatic potentials and spatial scales of a few Debye
lengths. The bipolar structures propagate highly oblique to the shock normal
with velocities (in the plasma rest frame) of the order of the ion-acoustic
velocity. We argue that the bipolar structures are ion phase space holes
produced by the two-stream instability between incoming and reflected ions.
This is the first identification of the ion two-stream instability in
collisionless shocks. The implications for electron acceleration are discussed.
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