Abstract
AbstractIn April 2011 Saturn's midlatitude ionospheric emissions were detected, exhibiting anomalous (nonsolar) 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 emissions recorded on 23 April 2013, showing for the first time since the original detection that Saturn's midlatitude 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 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.
Highlights
Prior to the arrival of any spacecraft at Saturn, early models describing Saturn’s ionosphere based on photoionization of H2, He, and H by extreme ultraviolet (EUV) radiation from the Sun predicted an electron density of around 1×105 cm−3 (McElroy, 1973; Waite et al, 1979)
The highest density was derived at a latitude of 73∘ south, while the lowest was found at 36∘ north, an unexpected latitudinal variation
The limb-tolimb distance in pixels (118 pixels) equates to 17 arc seconds, and together with the geometry obtained from planetary ephemeris (NASA’s Horizons web interface at https://ssd.jpl.nasa.gov/horizons.cgi), planetocentric latitudes were assigned
Summary
Prior to the arrival of any spacecraft at Saturn, early models describing Saturn’s ionosphere based on photoionization of H2, He, and H by extreme ultraviolet (EUV) radiation from the Sun predicted an electron density of around 1×105 cm−3 (McElroy, 1973; Waite et al, 1979). Kliore et al (1980) used the radio occultations to estimate the ionospheric electron density as a function of altitude, finding that the peak density was an order of magnitude less than models predicted, at 1×104 cm−3. Subsequent radio occultations by Voyager 1 and 2, in 1980 and 1981, showed peak electron densities between ∼6×103 cm−3 and ∼2.3×104 cm−3 (Atreya et al, 1984). Analyses of Saturn electrostatic discharges suggested dramatic variations of peak electron density with both latitude and local time Kaiser et al (1984)
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