Electric fields associated with small‐scale magnetic holes in the plasma sheet: Evidence for electron currents

We report observations of magnetic holes (MHs) in the near‐Earth (8 RE to 12 RE) plasma sheet that have physical sizes perpendicular to the magnetic field (B) on the order of the ion Larmor radius (ρi) and, more importantly, have current layers less than ρi in thickness. Small‐scale MHs can have >90% depletion in |B| and are commonly associated with the braking of bursty bulk flow events. The generation of MHs is often attributed to magnetohydrodynamic (MHD) instabilities, which requires a size greater than ρi; the depletion in |B| is from an ion current consistent with a pressure gradient. Electric field (E) observations indicate a negative potential inside of small‐scale MHs that creates an outward E at the boundary, which drives an E × B electron current in a thin layer. These observations indicate that a Hall electron current is primarily responsible for the depletion of |B| in small‐scale magnetic holes, rather than the ion pressure gradient.