Dawn‐dusk electric field asymmetry of the Io plasma torus

We consider the effects of a combined convection and corotation electric field across the Io plasma torus. A dawn‐to‐dusk electric field Ec will modify the orbits of charged particles shifting them toward dawn. The radial drift imposed by the perturbed orbits implies a local time‐dependent modulation of low‐energy ion and electron temperatures with particles hotter at dusk than at dawn. With Ec ≃ 4 mV/m, the orbits near 6 RJ would be shifted by approximately 0.2 RJ. Then the electron temperature would be 20% higher at dusk than at dawn, an effect which could explain the local time asymmetry of EUV intensity found by the Voyager ultraviolet spectrometer. The source of the convection electric field is internal to the magnetosphere and is attributed to the tailward escape of Iogenic and Jovian plasma beyond the Alfven surface.