Plasma sheet pressure evolution related to substorms

摘要

[1] We have investigated the inner equatorial plasma sheet plasma pressure evolutionassociated with substorm onsets using the five THEMIS spacecraft data during the2008 tail season. The central plasma sheet thermal pressure is estimated from the thermaland magnetic pressure based on one dimensional pressure balance. The results showthat for a majority of substorm cases, the plasma pressure enhances after onset earthwardof ~12 RE radial distance. On the other hand, the plasma pressure beyond —16 RE showsa remarkable decrease near onset. During the recovery phase, the pressure graduallyreturns to its original level. These results indicate that during the substorm expansionphase, the current wedge usually expands to the region tailward of —16 RE with the inneredge earthward of this radial distance. The pressure increase earthward of —12 RE could beassociated with an increase of cross-tail current, though this is not necessarily so due to thenontail-like magnetic field geometry after dipolarization. For a minority of cases, thepressure fluctuates without net change or reduces at —11 RE. During" these events, theground magnetic perturbations and the estimated center of the westward electrojet extendfurther equatorward than for the pressure increase cases. There is also evidence thataurora breakup occurs further equatorward when there is a pressure reduction, indicating amore earthward penetration of the plasma sheet. During these cases, the current wedgerepresented by pressure reduction is clearly observed earthward to —11 RE. Therelationship between the auroral break up arc, which is believed to map to —10 RE, andmay be associated with a precursor pressure enhancement, and the inner plasma sheetpressure and current evolution near onset warrants more careful analysis.