A source of auroral electrons and the magnetospheric substorm current systems

Details A source of auroral electrons and the magnetospheric substorm current systems A source of auroral electrons and the magnetospheric substorm current systems

Feb 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003jgra..108.8006a&link_type=abstract

Journal of Geophysical Research (Space Physics), Volume 108, Issue A4, pp. COA 7-1, CiteID 8006, DOI 10.1029/2002JA009547

Physics

15

Ionosphere: Auroral Ionosphere (2704), Ionosphere: Current Systems (2708), Magnetospheric Physics: Storms And Substorms, Magnetospheric Physics: Magnetosphere/Ionosphere Interactions

Scientific paper

Auroral electrons are field-aligned current carriers and thus changes of their flux are related to the magnetospheric substorm current systems. Therefore a study of processes associated with substorms can provide insight into a source of auroral electrons. This paper attempts to identify basic processes associated with substorm onset by synthesizing/integrating the three well-established observations at substorm onset: a sudden brightening of an auroral arc, a sudden growth of the westward electrojet, and dipolarization. It is demonstrated that when these observations are combined, they reveal basic processes associated with substorms and a source of auroral electrons. First of all, it is shown that, together with the southward-directed electric field Es, the associated Pedersen current Ip and its equatorial closing component Ir, the pair of the upward/downward field-aligned current sheets in the night sector forms a meridional current system (Boström's Case 2 current system); since Es . Ip > 0, driving electric field Ed must be present to drive Ir on the equatorial plane, so that Ed . Ir < 0. Thus, an enhancement of Boström's Case 2 current system can increase the upward current sheet, explaining a sudden brightening of auroral arcs. Also, it is noted that Ed drives the westward electrojet, which is mainly the Hall current. It is argued that the westward electrojet cannot be the diverted cross-tail current, and the dipolarization must be caused by the ``return current'' from the westward electrojet; the return current must be located in the equatorial plane of the magnetosphere. In fact, the diversion of the cross-tail currents explains little about the aurora. Further, the dipolarization sometimes results in an overdipolarization and even a negative dipolarization, which cannot be explained by the diversion of the cross-tail current. Therefore there must be an internal dynamo process that enhances Boström's current system on the equatorial plane. This explains many aspects of auroral arcs during substorms and provides a source of auroral electrons. The enhanced internal dynamo process during the expansive phase must be closely related to a short-lived unloading process. The disruption (not the diversion) of the cross-tail current can release magnetic energy stored during the growth phase for the unloading process. A chain of processes associated with the substorm warrants further study.

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