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

Abstract We report observations of magnetic holes (MHs) in the near‐Earth (8 R E to 12 R E ) plasma sheet that have physical sizes perpendicular to the magnetic field ( B ) on the order of the ion Larmor radius ( ρ i ) and, more importantly, have current layers less than ρ i 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 ρ i ; 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.