Response of the equatorial and polar magnetosphere to the very tenuous solar wind on May 11, 1999

We examine effects in the equatorial and polar magnetosphere during 9–13 May, 1999. Earth\u27s field at geostationary orbit became closely dipolar for ∼16 hours when solar wind densities nsw were \u3c1 cm−3. Electron precipitation in the northern polar cap intensified as nsw decreased, with significant fluxes up to ∼15 keV energy on May 11. The simultaneous precipitation void in the southern polar cap implies a very pronounced north-south asymmetry, also reflected in the hemispherical power deposition. With an intense and collimated strahl, these observations support the ideas of Fairfield and Scudder [1985] on the preferential entry of the strahl into the northern hemisphere under the observed IMF away sector as a source of the north-south precipitation asymmetry. The polar rain north-south asymmetry argues against an ejecta source for the solar wind drop-out. The temporal profiles of solar wind parameters were very asymmetric with respect to the time of minimum nsw, and strong compressions and substorm activity prevailed as nsw recovered