Local time asymmetry of the equatorial current sheet in Jupiter's magnetosphere

We provide a first systematic comparison of the radial fields associated with the equatorial current sheet in the Jovian magnetosphere which were observed during the flybys of the Pioneer-10 and -11, Voyager-1 and -2, and Ulysses spacecraft. These data span a similar to 210 degrees range of azimuths about the planet, from dusk via noon to the post-midnight sector. We show that these fields are systematically weaker on the dayside than on the nightside at distances beyond similar to 20 R-J, and fall more rapidly with jovicentric distance in the former regime than in the latter. This effect is significantly larger than, and not masked by, any secular changes in the current sheet strength associated, e.g. with changes in the Io gas production rate. Fits to the observed current sheet radial fields suggest approximate azimuthal symmetry at similar to 20 R-J Beyond this, the radial field outside the current sheet falls approximately as a power-law of the distance, with an exponent of similar to0.8 near midnight, increasing to similar to1.7 near noon. Consequently, the radial field at noon shows an increasing deficit relative to that at midnight at the same distance, reaching a factor of similar to2 at distances of similar to 40-50 R-J (generally corresponding to the outer region of the current sheet on the dayside). A simple model of the radial held outside the current sheet is presented which describes these effects as a function of radial distance and local time. We finally note that these results imply a significant divergence of the azimuthal equatorial current. In the radial distance range 20-50 R-J, the total deficit at noon compared with midnight computed from the model is similar to 34 MA. It is not known at present whether current continuity is maintained via the radial current in the current sheet, or via field-aligned currents coupling to the Jovian ionosphere, depending on the physical origin of the asymmetry. (C) 2001 Elsevier Science Ltd. Ail rights reserved.