Large scale response of the magnetosphere to a southward turning of the interplanetary magnetic field

Abstract

A set of coordinated measurements obtained on March 4, 1979, in the solar wind, in the magnetotail, at geostationary orbit, in the auroral ionosphere and on the ground allowed a detailed study of the overall magnetospheric response to a southward turning of the interplanetary magnetic field. This study shows that the magnetosphere responds without any appreciable delay to the change of the interplanetary magnetic field (IMF) direction. The intensity of the electrojets increases, and both westward electrojet and auroral forms move southward in the night sector. The magnetic field topology in the inner nightside magnetosphere (6.6 R E ) becomes increasingly taillike, inducing the adiabatic earthward motion of the high‐energy electrons and the precipitation of those with small equatorial pitch angles. All these events coincide with the increase of the solar wind‐magnetosphere energy coupling function ε. The large set of available data shows that a series of weak, short‐lived substorms sporadically occurs during the period of southward directed IMF, before the main substorm onset. These small events are characterized by all or part of the following features: partial plasma sheet flux dropout, weak, short‐lived, dipolar reconfiguration of the B field at 6.6 R E , impulsive changes of the ionospheric electric field, limited motions of the westward electrojet boundaries, injection of energetic particles and Pi 2 bursts. Finally a well‐defined substorm occurs about 80 min after the IMF reversal and during the southward IMF episode. Magnetic and auroral activity increases again when the IMF begins to turn northward and subsides as soon as the IMF B Z component reaches positive values. This study suggests that, for the March 4 southward IMF episode, the bulk of the energy dissipated in the inner magnetosphere and auroral ionosphere before the main substorm onset was directly extracted from the solar wind.