Magnetopause structure and the role of reconnection at the outer planets

Abstract

In situ measurements have been obtained at the magnetopause boundary of all the giant outer planets. The Jovian magnetopause was probed by Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, and most recently by Galileo. Saturn was visited by Pioneer 11 and Voyagers 1 and 2, and Uranus and Neptune were visited by Voyager 2. The observations at Jupiter show evidence for flux transfer event (FTE) structures and rotational discontinuities associated with magnetic reconnection with the interplanetary magnetic field (IMF), but in accord with previous studies we find no FTE signatures beyond Jupiter in the data sets we studied. At Saturn, one magnetopause encounter shows evidence for reconnection in the form of a rotational discontinuity with a finite magnetic field component through the boundary and significant plasma acceleration. At Uranus and Neptune the flank magnetopause boundary observed by Voyager 2 exhibits a complex structure (similar to that seen at Earth during high‐β conditions) for which unsteady reconnection appears to occur. Closely spaced multiple magnetopause crossings at Jupiter and Saturn are consistent with a boundary surface disturbance or wave, occurring with unsteady reconnection, and coincident with a jump in upstream solar wind dynamic pressure in the Jupiter case. Because of the transient, bursty nature of the reconnection signatures and their low (∼0.1 to 0.4 mV/m) convective electric fields compared to those of corotation at Jupiter and Saturn, the role of reconnection in driving the dynamics of these magnetospheres is thought to be minimal in the general case. Nevertheless, the unsteady reconnection observed is important to the properties of the boundary layers. At Uranus and Neptune, observations are limited (Voyager 2 only) but suggest that bursty reconnection at the flank magnetopause can only remove plasma (in the antisunward direction), and infrequent reconnection on the dayside would provide little opportunity for solar wind entry to energize the ionospheric plasma and supply the magnetosphere; these factors may contribute to producing the relatively empty magnetospheres that were seen.