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

Saturn's ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation(1). 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(2). The observed electron density of the low-latitude ionosphere, however, is lower and its temperature higher than predicted by models(3-5). 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(6-8). 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.