Cassini Radio Occultation Observations of Saturn's Ionosphere: Electron Density Profiles From 2005 to 2013

A set of electron density profiles of Saturn's ionosphere acquired by Cassini radio occultations is archived at the NASA Planetary Data System. However, the reference surface that defines zero altitude in these profiles is unknown and appears to vary by 1,500 km at fixed latitude. These profiles are immensely valuable for addressing questions pertaining to the vertical structure, meridional structure, diurnal variations, or solar cycle dependence of Saturn's ionosphere, but their value is severely limited by their questionable altitude scales. Here we have resolved this problem by independently generating the set of 60 electron density profiles. These profiles confirm that, as noted by previous authors, the structure of Saturn's ionosphere is highly variable. Nevertheless, the profiles suggest an underlying morphology of a broad layer at relatively high altitude (often at approximately 2,000-3,000 km altitude) and a series of narrower layers at lower altitude (often at approximately 1,000 km altitude). The vertical structure of Saturn's ionosphere depends on latitude and local time. At low latitudes, densities are greater at dusk than at dawn. Conversely, at mid latitudes, densities are greater at dawn than at dusk. The plasma scale height of the topside ionosphere is relatively small at low latitudes. The high, broad ionospheric layer is apparent at mid and high latitudes at both dawn and dusk, but is not present at low latitudes at either dawn or dusk. Total electron content also shows a strong dependence on latitude, with high latitudes having greater values than low and mid latitudes.