The domination of Saturn’s low-latitude ionosphere by ring ‘rain’

Abstract

Saturn's ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation. At low latitudes the solar radiation should result in a weak planet-wide glow in the infrared, corresponding to the planet's uniform illumination by the Sun. The observed electron density of the low-latitude ionosphere, however, is lower and its temperature higher than predicted by models. A planet-to-ring magnetic connection has been previously suggested, in which an influx of water from the rings could explain the lower-than-expected electron densities in Saturn's atmosphere. Here we report the detection of a pattern of features, extending across a broad latitude band from 25 to 60 degrees, that is superposed on the lower-latitude background glow, with peaks in emission that map along the planet's magnetic field lines to gaps in Saturn's rings. This pattern implies the transfer of charged species derived from water from the ring-plane to the ionosphere, an influx on a global scale, flooding between 30 to 43 per cent of the surface of Saturn's upper atmosphere. This ring 'rain' is important in modulating ionospheric emissions and suppressing electron densities.

Auroral Processes at the Giant Planets: Energy Deposition, Emission Mechanisms, Morphology and Spectra

“…Some of the water produced at Enceladus is lost to Saturn, mostly in the equatorial region (Fleshman et al 2012). Though part of the water influx is neutral, there is recent evidence for water ion precipitation at low-and mid-latitudes (O'Donoghue et al 2013). At high latitudes, the increase in H 2 (ν ≥ 4) (Cravens 1987) results in a more efficient removal of H + through charge-exchange with H 2 (see Section 2.2.1.2).…”

“…The intensity of the observed H + 3 emission reveals the morphology of the auroral deposition, which directly relates to where particle precipitation is sourced from in the magnetosphere (e.g. Connerney et al 1998;Badman et al 2011b;O'Donoghue et al 2013). 2.…”

Planetary Magnetism

“…The B-ring sources are identified with an electromagnetic erosion mechanism that sculpted the rings (Northrop and , a process that over tens of millions of years removed most of the mass from the C ring, for example. Recent Keck observations of H 3 + emissions from Saturn's ionosphere (O'Donoghue et al, 2013) reveal a latitudinal variation in ion density that suggests transport of water in the form of highcharge-to-mass-ratio particles along magnetic field lines from sources in the rings (Connerney, 2013). The Enceladus/E-ring source may reflect persistent activity of the recently discovered geysers on Enceladus (Kargel, 2006;Kivelson, 2006).…”

Discovery of suprathermal Fe+ in Saturn's magnetosphere

“…Such meteoric origin layers have not as yet been definitively observed at Saturn or Jupiter, but early analyses of Cassini data showing likely low‐ionospheric layered structure at Saturn is encouraging [ Molina‐Cuberos et al , ; Nagy et al , ]. Using Mg as a representative meteoric metal (other meteoric species include Na, Fe, C, and Si), Moses and Bass [] modeled meteoric layers at Saturn, addressing, among other effects, the recently documented “ring rain,” i.e., water influx from the main rings to Saturn's midlatitude atmosphere [ O ' Donoghue et al , ]. Given that meteoric layers exist at Saturn, ionospheric mixing, such as that at Earth (see e.g., Grebowsky and Aikin []), can spread some metal layer ions throughout the ionosphere.…”