Other
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agusmsm22a..03k&link_type=abstract
American Geophysical Union, Spring Meeting 2009, abstract #SM22A-03
Other
2407 Auroral Ionosphere (2704), 2721 Field-Aligned Currents And Current Systems (2409), 2790 Substorms
Scientific paper
The M-I coupling medium prior to substorm onset is reasonably characterized by a large-scale field aligned current (FAC) sheet (or sheets) coupling the magnetosphere and ionosphere, plus earthward convection, or equivalently, a quasi-steady large-scale dawn-dusk electric field Ed-d. Imagine now an enhancement in the FAC. This has the effect of displacing the current-carrying field lines in longitude, meaning in the direction of Ed-d, causing electric potential to vary along B. Ideally, these field-aligned potential gradients would be nullified immediately by currents parallel to B. However, finite electron mass not only allows field-aligned electric fields within convecting current sheets to persist indefinitely, the resulting FAC enhancements tend to reinforce the initial perturbation. A non-linear two-fluid model of this system [ Knudsen, J. Geophys. Res., 101, 10761, 1996] shows that perturbations in field-aligned potential drop, electron energy and FAC intensity grow in the direction of the convection flow, then saturate nonlinearly and return to their undisturbed value further downstream. As these oscillations represent strong enhancements in electron energy flux, they would be visible as spatially periodic enhancements in auroral brightness. In the absence of any resistivity mechanism other than electron inertia, brightenings recur with a spatial period that can range from one to tens or even hundreds of electron inertial lengths, meaning hundreds of meters to tens of km in the direction perpendicular to B when projected to ionospheric heights. Importantly, this spatial structure is determined by intrinsic properties of the M-I coupling medium, and not by a distant source. This is one aspect that distinguishes these "stationary inertial Alfvén waves" (StIAW) from conventional, time- oscillating inertial Alfvén waves, another being that electron acceleration within StIAW can reach many times the Alfvén speed in principle. While these structures have been proposed as explanations for pre-breakup, quasi-static auroral arcs, one can expect that they would intensify and multiply rapidly within the drastically enhanced FAC and convection that characterize substorm breakup.
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