Substorm onset by new plasma intrusion: THEMIS spacecraft observations

Abstract

Recent observations from the 2‐D array of high‐resolution THEMIS all‐sky imagers (ASIs) during the 2008 tail season have shown that the majority of substorm onsets are preceded by equatorward extension of an auroral arc initiated at the poleward boundary of the aurora zone, and that the aurora breakup is likely triggered when this precursor arc reaches the onset region. These observations were used to suggest that substorm onset is driven by a new population of plasma having lower global entropy content than that of the preexisting plasma intruding into the inner plasma sheet. We have taken advantage of conjunctions between the location of the precursor arc observed by the THEMIS ASI and THEMIS spacecraft and identified four events where clear signatures of the proposed new plasma intrusion were observed simultaneously in both the magnetotail and ionosphere. In two of the events, the intrusion plasma was captured more than 10 min before substorm onset in the midtail plasma sheet. The intruding plasma is characterized as the reduction of the pressure and entropy parameter associated with more dipolarized magnetic field, as well as the enhancement of the duskward electric field and earthward flow. In two other events, the plasma intrusion was found within 2 min before onset in the inner plasma sheet region R ∼11 RE, which is very close to the onset locations. These observations are consistent with the proposal that intrusion of low‐entropy content plasma to the onset region may provide a trigger for substorm onset. We also found two plasma intrusion events not associated with substorm onset in the plasma sheet, which indicates that the new plasma intrusion is not a sufficient condition for substorm onset. We suggest that sufficient pressure and azimuthal pressure gradient buildup in the inner plasma sheet during growth phase should be another necessary condition.