Magnetic energy storage and the nightside magnetosphere-ionosphere coupling

The change in the magnetic energy stored in the Earth's magnetotail for the January 14-15 1988 magnetic cloud event is investigated using an empirical magnetic field model. The results are used to calculate the two normal modes contained in the low-dimensional global model called WINDMI for the solar wind driven magnetosphere-ionosphere system. For that event, we find that the storage of magnetic energy in the geotail increases most rapidly (by 17%) in the phase where the IMF turns from northward to the dawn-dusk direction and then by an additional 5% in the phase where the IMF rotates from dawn-dusk to southward. The coupling of the magnetosphere-ionosphere (MI) through the nightside region 1 current loop transfers power to the ionosphere through two modes: a fast (period of minutes) oscillation and a slow (period of one hour) geotail cavity mode. The solar wind drives both modes in the substorm dynamics.