Sub-Proton Scale Magnetic Holes: Turbulence Simulations, Theory and Cluster Observations in the Earth's Plasma Sheet.

Abstract:

We investigate the properties of a new type of nonlinear coherent structure, called electron vortex magnetic holes (EVMHs) discovered during 2-D full particle realistic mass ratio simulations of turbulence. These structures form via the interaction of magnetic depletions and the local electron population in the plasma. They are characterised by local depressions in the magnetic field strength with circular cross-sections. We use PIC and test particle simulations to show that the magnetic structure is maintained by a current carried by an electron vortex which is due to a population of electrons with pitch angles close to 90 degrees in trapped, or quasi-trapped, non-adiabatic orbits. We then compare these results to Cluster observations of electron-scale magnetic holes (MH) in the Earth's plasma sheet. MHs in the solar wind are often explained in terms of the mirror mode instability. Here, in the plasma sheet, we show MH events in mirror stable environments, with high electron perpendicular temperature anisotropy, which cannot be explained in terms of a standard mirror mode growth. We show that EVMHs may provide a theoretical explanation for a majority of the MHs observed in the plasma sheet at scales less than the proton thermal gyroradius.