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Ann. Geophys., 19, 633-641, 2001
www.ann-geophys.net/19/633/2001/
© European Geosciences Union 2001
Astrid-2 and ground-based observations of the auroral bulge in the middle of the nightside convection throat
G. T. Marklund1, T. Karlsson1, P. Eglitis2, and H. Opgenoorth2
1Division of Plasma Physics, Alfvén Laboratory, Royal Institute of Technology, SE 10044 Stockholm, Sweden
2Swedish Institute of Space Physics, Uppsala, Sweden
Abstract. Results concerning the
electrodynamics of the nightside auroral bulge are presented based on
simultaneous satellite and ground-based observations. The satellite data
include Astrid-2 measurements of electric fields, currents and particles from a
midnight auroral oval crossing and Polar UVI images of the large-scale auroral
distribution. The ground-based observations include STARE and SuperDARN
electric fields and magnetic records from the Greenland and MIRACLE
magnetometer network, the latter including stations from northern Scandinavia
north to Svalbard. At the time of the Astrid-2 crossing the ground-based data
reveal intense electrojet activity, both to the east and west of the Astrid-2
trajectory, related to the Polar observations of the auroral bulge but not
necessarily to a typical substorm. The energetic electron fluxes measured by
Astrid-2 across the auroral oval were generally weak being consistent with a
gap observed in the auroral luminosity distribution. The electric field across
the oval was directed westward, intensifying close to the poleward boundary
followed by a decrease in the polar cap. The combined observations suggests
that Astrid-2 was moving close to the separatrix between the dusk and dawn
convection cells in a region of low conductivity. The constant westward
direction of the electric field across the oval indicates that current
continuity was maintained, not by polarisation electric fields (as in a Cowling
channel), but solely by localized up- and downward field-aligned currents in
good agreement with the Astrid-2 magnetometer data. The absence of a
polarisation electric field and thus of an intense westward closure current
between the dawn and dusk convection cells is consistent with the relatively
weak precipitation and low conductivity in the convection throat. Thus, the
Cowling current model is not adequate for describing the electrodynamics of the
nightside auroral bulge treated here.
Key words. Ionosphere (auroral
ionosphere; electric fields and currents; plasma convection)
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