Azimuthal Pressure Gradient and Associated Auroral Development in the Near-Earth Plasma Sheet Soon Before Substorm Onset

The azimuthal plasma pressure gradient in the near-Earth plasma sheet makes crucial contributions to field-aligned current (FAC) formation. Numerical simulations and statistical observations have shown that a plasma pressure peak tends to build up in the pre-midnight region of the near-earth plasma sheet during the substorm growth phase due to enhanced magnetic drift. This leads to azimuthal pressure gradients in this region. The temporal variation of the azimuthal pressure gradient may provide an indication for the FACs variations associated with the substorm growth phase and thus may setup a plasma sheet pre-condition for the substorm onset being triggered near this region. We take advantage of two of the THEMIS spacecraft separated azimuthally near the orbit apogee and investigate the azimuthal plasma pressure gradient in the R~10-12 RE region. Equatorial plasma pressure is estimated by removing the curvature force effect. Four events with the spacecraft footprints mapped very close to the aurora onset region were selected. Two of them showed substantial duskward pressure gradient enhancement 2-5 min before onset. A late-growth-phase auroral arc, which became the substorm onset arc, was found to intensify simultaneously with the pressure gradient enhancement. The other two vents showed almost no change of azimuthal pressure gradient before onset, at the same time neither growth-phase auroral arc intensification was found near the spacecraft footprints, nor did onset occur along an observable arc. These results indicate that the duskward azimuthal pressure gradient enhancement associated with enhanced upward FACs during the late-growth-phase may provide important energy redistribution for the intensification of the growth-phase auroral arc soon before it breaks up and set up conditions for substorm onset to occur along such an arc.