A dawn-to-dusk electric field in the Jovian magnetosphere

We show that if Io-injected plasma is lost via a planetary wind a sun-fixed Birkeland current system may result. This is due to the fact that a current flows across a density gradient produced by the loss of plasma through the planetary wind in the tail. The divergent current is connected to field-aligned Birkeland currents which flow into the ionosphere at dawn and out of it at dusk. The closure currents in the ionosphere require a dawn-to-dusk electric field which at the orbit of Io is estimated to have a strength of a few mV m^-1. Independent estimates derived from the local time asymmetry of the torus u.v. emission indicate a field of 1.5mVm^-1.