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 Ma y, 1999. Earth's field at geostationary orbit became closely dipolar for si milar to 16 hours when solar wind densities n(sw) were < 1 cm(-3) Electron precipitation in the northern polar cap intensified as n(sw) decreased, wit h significant fluxes up to similar to 15 keV energy on May 11. The simultan eous precipitation void in the southern polar cap implies a very pronounced north-south asymmetry, also reflected in the hemispherical power depositio n. With an intense and collimated strahl, these observations support the id eas of Fairfield and Scudder [1985] on the preferential entry of the strahl into the northern hemisphere under the observed IMF away sector as a sourc e of the north-south precipitation asymmetry. The polar rain north-south as ymmetry argues against an ejecta source for the solar wind drop-out. The te mporal profiles of solar wind parameters were very asymmetric with respect to the time of minimum n(sw), and strong compressions and substorm activity prevailed as n(sw) recovered.