Nature of the ring current in Saturn's dayside magnetosphere

摘要

We employ magnetic field and plasma particle data from two equatorial passes of the Cassini spacecraft spanning the radial range between ～3 and ～20 Rs to investigate the nature of the ring current in Saturn's dayside magnetosphere (R_5 is Saturn's equatorial radius, equal to 60,268 km). We examine plasma parameters obtained by Cassini for each pass and compare the azimuthal current density profiles deduced with those obtained from current disk modeling of the magnetic field perturbations. We show that the current associated with the P>P_∥ pressure anisotropy of the warm water group ions is important inside ～10 R_s, canceling a significant fraction of the otherwise dominant inertia current inside ～6 R_s. The overall total current density profile is then found to be similar to that produced by the pressure gradient current but augmented in strength by factors of ～ 1.5-2.0 by the difference between the inertia and anisotropy currents. The total current density rises from small values near ～6 R_s, peaks at ～100 pA M~(-2) near～8 R_s, and reduces to values below ～25 pA m~(-2) at distances beyond ～15 R_s, up to the 20 R_s limit of our study. The deduced current density profiles are in good agreement with the gross features of the profiles deduced from current disk modeling, with peak values occurring just outside the inner edge of the best fit models at ～7 R_s, though with the current density then falling more steeply with radial distance than the l/γ dependence assumed in the model.