Cassini observations of Saturn's southern polar cusp

The magnetospheric cusps are important sites of the coupling of a magnetosphere with the solar wind. The combination of both ground‐ and space‐based observations at Earth has enabled considerable progress to be made in understanding the terrestrial cusp and its role in the coupling of the magnetosphere to the solar wind via the polar magnetosphere. Voyager 2 fully explored Neptune's cusp in 1989, but highly inclined orbits of the Cassini spacecraft at Saturn present the most recent opportunity to repeatedly study the polar magnetosphere of a rapidly rotating planet. In this paper we discuss observations made by Cassini during two passes through Saturn's southern polar magnetosphere. Our main findings are that (i) Cassini directly encounters the southern polar cusp with evidence for the entry of magnetosheath plasma into the cusp via magnetopause reconnection, (ii) magnetopause reconnection and entry of plasma into the cusp can occur over a range of solar wind conditions, and (iii) double cusp morphologies are consistent with the position of the cusp oscillating in phase with Saturn's global magnetospheric periodicities. Key Points Saturn's southern polar cusp is active under a range of solar wind conditions Evidence for injection of magnetosheath plasma into the cusp via dayside magnetopause reconnection The position of Saturn's southern polar cusp oscillates in with global magnetospheric periodicities