Storm time dawn-dusk asymmetry of the large-scale Birkeland currents

Abstract

Magnetometer data from the Iridium constellation of satellites are used to characterize storm-time global Birkeland currents. The geomagnetic storm of 1 October 2002 was driven by a slowly rotating interplanetary magnetic field (IMF) and the currents display a 3° latitude equatonvard shift at dusk relative to dawn and have higher intensity at dusk. The asymmetry develops during the storm main phase and lasts to the early recovery phase, corresponding to the time of strongest ASY-H. A survey of 37 large storms, Dst minimum below - 100 nT, from February 1999 through December 2004 yielded 62 two-hour intervals from 22 storms for which the Iridium data indicated sufficiently steady conditions to warrant calculation of the Birkeland currents. Statistical analysis shows that the storm-time dusk currents are shifted equatonvard relative to dawn by 2.4° on average with a range of to -2.9° to 6.9°. The latitude asymmetry depends on two independent factors. First, the latitude asymmetry correlates with the maximum magnetic perturbation at noon, indicating a shift of high-latitude convection toward dawn (dusk) for positive (negative) IMF By. Second, the dusk currents shift ∼1° further equatorward and intensify ∼0.06 MA/° per 100 nT in ASY-H more than the currents at dawn. We suggest that the ion pressure associated with the partial ring current leads to stronger duskside currents and an asymmetric inflation of the magnetosphere, shifting the ionospheric projection of currents at dusk to lower latitudes. Copyright 2005 by the American Geophysical Union.