Divergence of the equatorial current in the dawn sector of Jupiter's magnetosphere: analysis of Pioneer and Voyager magnetic field data

Abstract

Averaged values of the azimuthal component of the magnetic field observed outside the jovian middle magnetosphere equatorial current sheet have been used to derive radial profiles of the radial current intensity over the jovicentric distance range 20–50 R J. Data from four spacecraft flybys have been used, spanning the dawn sector from ∼0100 to ∼0900 MLT (i.e. inbound Pioneer-11, and outbound Pioneer-10, and Voyagers-1 and -2). These profiles have been combined with a recent empirical model of the azimuthal current intensity to estimate the total divergence of the current in the current sheet along the trajectory, and hence the density of the field-aligned current that couples the current sheet and the ionosphere. For the Voyager passes the inferred field-aligned current flows from the ionosphere into the current sheet, with ( j ||/ B)≈2– 10×10 −13 A m −2 nT −1 . The inferred field-aligned currents at the ionosphere require significant field-aligned acceleration of thermal magnetospheric electrons (of 2–3 keV thermal energy), through voltages of ∼40–150 kV. The latitude of these currents, together with the estimated electron energy flux deposited in the ionosphere (0.01–0.1 W m −2), suggests a direct connection with the main jovian auroral oval. Related currents are present on the Pioneer passes as well, but lie equatorially inside of ∼20 R J. Between 20 and 50 R J, the inferred field-aligned currents on these passes are either small (Pioneer-11), or reversed in sense with similar densities (Pioneer-10). The radial profiles of the radial current associated with magnetosphere–ionosphere coupling have also been used to derive radial profiles of the angular velocity of the magnetospheric plasma, for given values of the effective Pedersen conductivity of the jovian ionosphere. Reasonable profiles are obtained for effective conductivities of several tenths of a mho. The Voyager data then indicate slowly falling values from near-rigid corotation at ∼20 R J, to ∼50% of rigid corotation at ∼50 R J. For the same values of the conductivity the Pioneer data indicate smaller angular velocities in the inner region (∼70% of rigid corotation), remaining either constant or even increasing with distance. This behaviour may either indicate the presence of some magnetospheric dynamic process operating in the outer magnetosphere on these passes, or could alternatively be due to falling conductivities with distance in the conjugate ionosphere.