Abstract
<p>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.</p>
Highlights
IntroductionSaturn's magnetosphere is a dynamical environment, shaped by the solar wind, the planet's fast rotation, a strong planetary magnetic field, and significant mass-loading from the moon Enceladus (Gombosi et al, 2009; Thomsen et al, 2013, and references therein)
During solar wind compressions when the planetary period oscillations (PPOs) are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection
We have examined the nightside ring current as observed on 21 proximal periapsis passes during Saturn's Grand Finale
Summary
Saturn's magnetosphere is a dynamical environment, shaped by the solar wind, the planet's fast rotation, a strong planetary magnetic field, and significant mass-loading from the moon Enceladus (Gombosi et al, 2009; Thomsen et al, 2013, and references therein). The form of Saturn's magnetosphere is determined by pressure balance at the magnetopause boundary between the internal stresses and the solar wind dynamic pressure (e.g. Russell et al, 2003). The action of centrifugal forces confines magnetospheric plasma towards the equatorial plane, resulting in the formation of a disc-like distribution of plasma through which a ring current flows in an azimuthal eastward direction. The solar wind bends Saturn's dayside and nightside ring current into a bowl shape, which was tilted south of the equatorial plane during the northern hemisphere summer conditions studied in this paper (Arridge et al, 2008b; Bunce et al, 2008; Cowley et al, 2006—see Equation 6 below)
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