A Kinetic M-i Coupling Model with Unloading Instability at Onset of Substorm

Onset of substorm due to unloading instability is proposed in this paper based on a kinetic M-I coupling model. Nonadiabatic motion of energetic ions in the near Earth plasma sheet can lead to an effective Hall conductivity in the plasma sheet similar to the Hall conductivity in the E-region ionosphere. Hall conductivity can lead to left-hand rotation of electric field with respect to the background magnetic field. Nonuniform distribution of diffuse aurora in ionosphere and nonuniform distribution of energetic ions in plasma sheet can result in nonuniform distribution of Hall conductivity. Nonuniform Hall conductivity can lead to nonuniform rotation of electric field. As a result of M-I coupling process, a southward curl-E field distribution can be found in the premidnight region of near Earth plasma sheet prior to the onset of substorm. An electron time scale dynamo process in response to the southward curl-E field distribution can lead to formation of premidnight upward field-aligned current. The premidnight upward field-aligned current can initiate dipolarization in the midnight region and trigger unloading instability. Unloading instability will grow if there are enough energetic ions in the near Earth plasma sheet at onset of substorm. Enough energetic ions in the plasma sheet imply that the cross-tail current is mainly contributed by the Hall current. Positive feedback between thickening of current sheet and reduction of Hall current leads to amplification of current disruption and onset of substorm expansion phase. Our model can explain the observed magnetic field signature during explosive thinning prior to onset of substorm. Our model can also explain onset of substorm due to northward turning of IMF.