Dayside auroral configurations: Responses to southward and northward rotations of the interplanetary magnetic field

P. E. Sandholt, C. J. Farrugia, J. Moen, and S. W. H. Cowley

Department of Physics, University of Oslo, Norway


We report specific types of daytime auroral activities occurring in response to certain rotations of the interplanetary magnetic field (IMF). The IMF rotations we focus on consist of a sharp southward turning from an initial northward orientation, followed by slower rotations back to an intermediate state, which is dominated by the By component (small Bz). When the IMF turned from north to south, the dayside auroral configuration changed from one typical of northward conditions to one typical of southward conditions. The type 2 cusp aurora in the north (~78° -79° magnetic latitude) was replaced by activation of a sequence of moving auroral forms (type 1 cusp aurora) at lower latitudes, each form appearing at the equatorward boundary of the previous one, resulting in ~100-to 200-km total displacements of the auroral equatorward boundary in the cusp region during intervals of ~5-to 10-min duration. This auroral activity is accompanied by enhancement of ionospheric convection and onset of magnetic bays on the ground. Since southward rotations of the IMF are expected to give rise to enhancements of the magnetopause reconnection rate, we conclude that these auroral/convection events represent distinct ionospheric signatures of such reconnection rate enhancements. When the IMF turned partially northward again, into an intermediate state, the aurora responded as expected, i.e., the equatorward auroral displacement was halted or reversed, and the type 2 aurora reappeared. The coexistence of the types 1 and 2 cusp auroras in the intermediate state is interpreted in terms of the simultaneous occurrence of reconnections at low and high magnetopause latitudes. The dynamical configurations of particle precipitation and ionospheric convection in the cusp region in such cases are discussed.

J. Geophys. Res., Vol. 103, No. A9, 20,279-20,295, 1998