Mid‐latitude reflection of ion upflows during substorm dipolarization

Abstract

Reconfiguration of the magnetospheric current system during the expansion phase of substorms leads to dipolarization of the magnetic field lines in the inner plasma sheet. This process is characterized by inward field line motion in the equatorial region and outward motion at mid‐latitudes. We investigate the parallel motion of ions during such dynamical reconfigurations. We demonstrate that, in contrast to equatorial latitudes where ions are centrifugally jetted toward the midplane and experience prominent energization, ions at mid‐latitudes are subjected to a downward (earthward) oriented acceleration. We accordingly show that ions with small parallel speeds such as those originating from the topside ionosphere may be decelerated upon reaching mid‐latitudes and return to low altitudes. Field line dipolarization during substorms may thus go together with substantial downward flow of low‐energy ions, as evidenced from measurements of the low‐altitude orbiting Interball‐Auroral spacecraft.