Physics
Scientific paper
Apr 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998georl..25.1269l&link_type=abstract
Geophysical Research Letters, Volume 25, Issue 8, p. 1269-1272
Physics
28
Magnetospheric Physics: Storms And Substorms, Magnetospheric Physics: Magnetosphere/Ionosphere Interactions, Magnetospheric Physics: Auroral Phenomena
Scientific paper
Two important observational constraints on substorm onset theories are that (1) substorm onset occurs on field lines of a pre-existing auroral arc, and (2) reduction of the cross-tail current occurs in the near-Earth magnetotail. Recent progress in linking generation of at least some auroral arcs to field line resonances (FLRs) and current disruption to possibly the cross-field current instability (CCI) suggests a synergistic combination of these two theories. We suggest that as the cross-tail current sheet thins to the extent that ions become unmagnetized at the end of substorm growth phase, the equatorial electric field associated with FLRs can amplify significantly the cross-tail current to exceed the instability threshold. This combined theory naturally accounts for the observational constraints of substorm onset starting on a pre-existing auroral arc and linking closely with current reduction in the near-Earth magnetotail. It is compatible with the observation of the most equatorward auroral arc being the most likely one for initial brightening because the cross-tail current is most intense in its near-Earth end and is therefore most likely to exceed the unstable threshold for CCI. It is also consistent with auroral precipitation fading to be a common feature before an auroral breakup. Other predictions yet to be tested are that (1) the onset is more favorable for the phase of the resonance in which the resonance-associated field-aligned current is directed from the ionosphere to the magnetosphere, and (2) the onset is more favorable poleward of the resonance shell or auroral arc.
Lui Anthony Tat Yin
Murphree John S.
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