Peak electron densities in Saturn's ionosphere derived from the low-frequency cutoff of Saturn lightning

Abstract

Radio bursts from Saturn lightning have been observed by the Cassini Radio and Plasma Wave Science instrument at frequencies of a few megahertz during several month-long storms since 2004. As the radio waves traverse Saturn's ionosphere on their way to the spacecraft, one can determine the peak electron density from the measurement of the low-frequency cutoff below which the radio bursts are not detected. In this way we obtained 231 profiles of peak electron densities that cover all Saturnian local times at a kronocentric latitude of 35°S, where the storms were spotted by the Cassini camera. Peak electron densities show a large variation at dawn and dusk and are around 5 × 10^4 cm^(−3), in fair agreement with radio occultation measurements at midlatitudes. At noon and midnight, the densities are typically somewhat above 10^5 cm^(−3) and around 10^4 cm^(−3), respectively. The diurnal variation is about 1 to 2 orders of magnitude for averaged profiles over one storm at 35°S. This is somewhat less compared to previous Voyager measurements which showed more than 2 orders of magnitude variation. The diurnal variation as well as the peak electron densities of Saturn's ionosphere tend to decrease with the decreasing solar EUV flux from 2004 until the end of 2009.