Current Events at Saturn: Ring–Planet Electromagnetic Coupling

Abstract

This study presents a synthesized analysis of in situ and ground-based observations to investigate electromagnetic coupling between Saturn and its rings. During the Cassini Grand Finale, the magnetometer detected gradients in the azimuthal magnetic field B _ϕ connected to Saturn’s B-ring on 17 out of 21 orbits. The B _ϕ gradients indicate that field-aligned currents are flowing into Saturn’s B-ring at ∼1.55–1.67 R _S in the ring plane, preferentially in the southern hemisphere. On average, these currents are magnetically conjugate with ground-based observations of nonsolar enhancements in ${{\rm{H}}}_{3}^{+}$ emissions from Saturn’s ionosphere and detected contemporaneously with ring-sourced, planetward electron beams and field-aligned charged dust grain inflow from the C- and B-rings into Saturn’s atmosphere. Collectively, these observations align with Voyager-era predictions of a phenomenon known as “ring rain,” where charged ring material generated inward of a nominal “critical radius” is drawn into Saturn’s upper atmosphere along the magnetic field. However, we show that the B-ring currents are not likely to be a direct signature of infalling field-aligned ring grains. Instead, we propose that the ring rain generation mechanism naturally results in a sharp gradient in the ionospheric Pedersen conductance at the ∼1.57–1.67 R _S boundary, which, combined with a v × B electric field in the ring ionosphere, could drive the observed B-ring currents. The Pedersen conductance in the high-conductance region of the southern ring ionosphere is constrained to ∼0.07–2 S and is observed to vary within this range on week-long timescales.

Keywords

• Planetary magnetospheres
• Planetary rings
• Planetary ionospheres
• Saturn