Substorm Onset by new Plasma Intrusion 3: THEMIS Spacecraft Observations

Details Substorm Onset by new Plasma Intrusion 3: THEMIS Spacecraft Observations Substorm Onset by new Plasma Intrusion 3: THEMIS Spacecraft Observations

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufmsm41b1710x&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #SM41B-1710

Physics

[2736] Magnetospheric Physics / Magnetosphere/Ionosphere Interactions, [2790] Magnetospheric Physics / Substorms

Scientific paper

In a companion study, observations from the two-dimensional array of high resolution THEMIS All Sky Imagers (ASI) during the 2008 tail season are used to show that the majority of substorms onsets are preceded by equatorward extension of an auroral arc initiating 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 suggest that substorm onset is driven by a new population of plasma having lower global entropy content than that of the pre-existing plasma intruding into the inner plasma sheet. We have taken advantage of the good longitudinal conjunctions between the location of the precursor arc observed by the THEMIS ASI and THEMIS spacecraft, and identified three events where clear signatures of the proposed new plasma intrusion are observed simultaneously in both the magnetotail and ionosphere. For the first event having a precursor arc that started to move toward the onset arc ~10 min before substorm onset, the lobe magnetic pressure near -17 RE reduced at approximately the same time and the duskward electric field underwent a substantial increase. These are expected signatures of a lower entropy content population of plasma appearing in the plasma sheet at that same radial distance. For the second event, as the precursor arc moved close to the onset arc ~2 min prior to onset, the lobe pressure near -11 RE was observed to decrease at the same time, which suggests the earthward penetrating of the new plasma population. For the third event, the azimuthal movement of the new plasma population is found to be consistent with the aurora precursor azimuthal motion observed in the ionosphere. 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. Further observations of plasma flows near the central plasma sheet are needed to confirm the identification of this plasma entry.

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