Saturn’s nightside ring current during Cassini’s Grand Finale

During Cassini’s Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ~21 Saturn radii.  Clear signatures of Saturn’s equatorial current sheet are observed in the magnetic field data.  An axisymmetric model of the ring current is fitted to these data, amended to taken into account the tilt of the current layer by solar wind forcing, its teardrop-shaped nature and the magnetotail and magnetopause fringing fields.  Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind, and internal driving by the two planetary period oscillations (PPOs) and compared with dawn and dayside regimes.  The relative phasing of the PPOs determines the ring current’s response to solar wind conditions. During solar wind compressions when the PPOS are in antiphase, magnetospheric storms are triggered and a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection.  However, during solar wind compressions when the PPOs are in phase, the magnetosphere shows only a ‘minor’ response and a partial ring current is not observed. During solar wind rarefactions an equatorial ‘magnetodisc’ configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times.  This partial ring current should close partly via magnetopause currents and possibly via field-aligned currents into the ionosphere.  During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.