Redetection of the Ionospheric Signature of Saturn's “Ring Rain”

Abstract

In April 2011 Saturn's midlatitude ionospheric normalH3+ emissions were detected, exhibiting anomalous (nonsolar) normalH3+ latitudinal variations consistent with the transport of water from specific locations in Saturn's rings, known as “ring rain”. These products, transported to the planet along the magnetic field, may help to explain the unusual pattern of peaks and troughs in electron densities discovered in Saturn's ionosphere by spacecraft flybys. In the present study, we analyzed normalH3+ emissions recorded on 23 April 2013, showing for the first time since the original detection that Saturn's midlatitude normalH3+ emissions are indeed heavily modified. Although the 2013 emissions are dimmer by almost a factor of 3.7, the latitudinal contrast is greater and uncertainties are lower. Increased normalH3+ intensities were found near planetocentric latitudes of 43°, 51°, and 63°, previously identified with sources at the inner edge of the B ring, A ring, and the orbit of Enceladus and associated E ring.

Electron Density Distributions in Saturn's Ionosphere

“…This interaction was theorized in early models to explain the lower peak electron densities observed in the Pioneer and Voyager radio occultations (Kliore et al, ; Lindal et al, ). The most popular explanation to reduce the light ion densities in early models to correlate with the radio occultation density measurements was the introduction of charged water particles from the rings into Saturn's midlatitude ionosphere, called ring rain (Connerney & Waite, ; O'Donoghue et al, ). O'Donoghue et al () detected latitudinal variations in the signature of midlatitude ionospheric H 3 + ions consistent with an influx of electrically charged water particles from the rings along magnetic field lines mapping to the inner edges of the B and A rings and the Enceladus orbit.…”

“…The most popular explanation to reduce the light ion densities in early models to correlate with the radio occultation density measurements was the introduction of charged water particles from the rings into Saturn's midlatitude ionosphere, called ring rain (Connerney & Waite, ; O'Donoghue et al, ). O'Donoghue et al () detected latitudinal variations in the signature of midlatitude ionospheric H 3 + ions consistent with an influx of electrically charged water particles from the rings along magnetic field lines mapping to the inner edges of the B and A rings and the Enceladus orbit. In situ measurements by Cassini's Ion and Neutral Mass Spectrometer (INMS), the Magnetosphere Imaging Instrument, and the RPWS/Langmuir Probe instrument observed an influx of heavy neutrals and dust grains from the rings into Saturn's ionosphere at equatorial latitudes (Mitchell et al, ; Waite et al, ).…”

Dust grains fall from Saturn’s D-ring into its equatorial upper atmosphere

“…The water-based ice grains in the atmosphere absorb electrons, thereby reducing the ionospheric electron density, primarily at latitudes that lie at the atmospheric feet of magnetic field lines connected to the rings (2). This reduction has been remotely observed from ionospheric occultation measurements (7,8) and from ground-based telescope observations (6). Simulations of ring rain suggest that charged dust precipitation should be greatest at the magnetic foot-points of the main rings (>15°from the equator), whereas the ionospheric electron densities are observed to be lowest near the equator (8)(9)(10).…”

“…In this region, magnetic drag on charged grains can slow them below local Keplerian orbital velocity, and the gravitational force dominates over the electromagnetic and centrifugal forces that otherwise keep such small (~100-nm-to 1-mm-sized) charged dust grains in orbit. The charged grains are then pulled along magnetic field lines into the planet's atmosphere (3)(4)(5)(6). The water-based ice grains in the atmosphere absorb electrons, thereby reducing the ionospheric electron density, primarily at latitudes that lie at the atmospheric feet of magnetic field lines connected to the rings (2).…”

In situ collection of dust grains falling from Saturn’s rings into its atmosphere

“…A fraction of the material released by erosion from the rings is expected to fall into Saturn's atmosphere as "ring rain" (16)(17)(18). Charged particles from the rings, in the form of ions or nanoparticles, gyrate and bounce along the planet's magnetic field lines and preferentially migrate into Saturn's southern atmosphere (19)(20)(21)(22)(23)(24)(25).…”