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Ann. Geophys., 20, 1049-1061, 2002
www.ann-geophys.net/20/1049/2002/
© European Geosciences Union 2002
Investigating the auroral electrojets with low altitude polar orbiting satellites
T. Moretto1, N. Olsen2, P. Ritter3, and G. Lu4
1Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
2Danish Space Research Institute, Copenhagen, Denmark
3Geoforschungs-Zentrum Potsdam, Potsdam, Germany
4High Altitude Observatory, Boulder, Colorado, USA
Correspondence to: T. Moretto
(Therese.Moretto@gsfc.nasa.gov)
Abstract. Three geomagnetic
satellite missions currently provide high precision magnetic field measurements
from low altitude polar orbiting spacecraft. We demonstrate how these data can
be used to determine the intensity and location of the horizontal currents that
flow in the ionosphere, predominantly in the auroral electrojets. First, we
examine the results during a recent geomagnetic storm. The currents derived
from two satellites at different altitudes are in very good agreement, which
verifies good stability of the method. Further, a very high degree of
correlation (correlation coefficients of 0.8–0.9) is observed between the
amplitudes of the derived currents and the commonly used auroral electrojet
indices based on magnetic measurements at ground. This points to the potential
of defining an auroral activity index based on the satellite observations,
which could be useful for space weather monitoring. A specific advantage of the
satellite observations over the ground-based magnetic measurements is their
coverage of the Southern Hemisphere, as well as the Northern. We utilize this
in an investigation of the ionospheric currents observed in both polar regions
during a period of unusually steady interplanetary magnetic field with a large
negative Y-component. A pronounced asymmetry is found between the
currents in the two hemispheres, which indicates real inter-hemispheric
differences beyond the mirror-asymmetry between hemispheres that earlier
studies have revealed. The method is also applied to another event for which
the combined measurements of the three satellites provide a comprehensive view
of the current systems. The analysis hereof reveals some surprising results
concerning the connection between solar wind driver and the resulting
ionospheric currents. Specifically, preconditioning of the magnetosphere
(history of the interplanetary magnetic field) is seen to play an important
role, and in the winther hemisphere, it seems to be harder to drive currents on
the nightside than on the dayside.
Key words. Ionosphere (electric
fields and currents) – Magnetospheric physics (current systems;
magnetosphere-ionosphere interactions)
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