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A substorm model with onset location tied to an auroral arc
Author(s) -
Lui A. T. Y.,
Murphree J. S.
Publication year - 1998
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/98gl00758
Subject(s) - substorm , magnetosphere , geophysics , ionosphere , physics , instability , arc (geometry) , breakup , field line , current (fluid) , electric field , plasma sheet , field (mathematics) , magnetic field , mechanics , geometry , mathematics , quantum mechanics , pure mathematics , thermodynamics
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.