Growth phase of Jovian substorms

Strong northward and southward field turnings in the near Jovian magnetotail have been observed by Galileo. These sudden dipolarizations and plasmoid formations have been interpreted as signatures of tail reconnection, similar to those seen in terrestrial substorms. In this paper, we examine the temporal behavior of the magnetic field strength in the lobes of the Jovian tail and find that it increases smoothly prior to reconnection events and decreases after them. The tail field variations are similar to those during the growth phase and expansion phase of terrestrial substorms but last a much longer time, about three days. These lobe field changes appear not to be caused by the solar wind dynamic pressure (P(dyn)). Thus substorms in the Jovian tail resemble terrestrial substorms in that they have growth phases with increases of the magnetic field strength in the near-planetary tail field followed by sudden decreases of the field strength when reconnection occurs and plasmoids are released.