Response of Uranus' auroras to solar wind compressions at equinox

We consider open flux production and magnetic tail formation for the unique physical circumstances at Uranus, where the planet's spin axis lies close to the orbit plane, while the magnetic dipole has a large ~60° inclination to the spin axis. Under these circumstances, open flux production and transport into the nightside can in principle occur continuously for near-solstice conditions, leading to the formation of a well-developed bipolar rotating magnetic tail, as observed during the Voyager 2 flyby in 1986. However, it is argued that tail formation will be significantly inhibited near to equinox, as open tubes are wound over the dayside magnetopause by planetary rotation, thus reducing further open flux production, and are compressed together north or south of the planet promoting “tail” reconnection and open flux closure. If so, this may account for the weak auroral responses to predicted solar wind compressions of Uranus' magnetosphere reported recently from Hubble Space Telescope observations under near-equinoctial conditions. Major auroral responses are observed following strong compressions at Earth and Saturn resulting from rapid and substantial closure of preexisting open flux in the tail. The lack of such a response at Uranus near equinox may thus reflect the lack of a well-developed tail under these conditions.