Thickness of Saturn's ring current determined from north-south Cassini passes through the current layer

摘要

We investigate magnetic field and plasma electron data from six Cassini orbits during which, unique within the mission to date, the spacecraft passed almost directly north-south through Saturn's equatorial plane within the ring current region of the middle magnetosphere. Three passes took place in the postnoon sector and three took place in the postmidnight sector, each at radial distances of ~9, ~12, and ~15 Rs. (Rs is Saturn's radius, equal to 60,268 km). These data allow for the first time an essentially direct determination of the north-south field and plasma structure in this region at approximately constant radius and local time. The dayside data indicate the presence of an equatorial current disk with a near constant half-thickness of ~4.5 Rs. Comparison of the magnetic data with simultaneous electron spectrograms shows that the current layer is located within a significantly broader layer of hot plasma. More variable conditions are found on the nightside. In the inner region a thin equatorial current layer of half-thickness ~ 0.5 RS is found embedded at the center of a much broader layer of current and plasma. In the central and outer passes the current sheet corresponds to essentially the whole of the hot equatorial electron layer, with a half-thickness of ~2.5 Rs for the central pass, falling strongly to ~0.4 RS for the outer pass. The data also provide confirmatory evidence of northward displacements of the center of the current layer from Saturn's equatorial plane in the outer regions, on both the dayside and the nightside of the planet.