UCL Discovery

Abstract

We analyze the radial distribution of electron populations inside 20 R-s in Saturn's magnetosphere, and we calculate moments for these populations by a forward modeling method using composite spectra produced by the CAPS/ELS (0.6 eV to 26 keV) and the MIMI/LEMMS (15 keV to 10 MeV) instruments on board Cassini. We first calculate and harmonize both data sets in physical units and apply corrections taking into account biases introduced by spacecraft interaction with the magnetospheric environment. We then test different bimodal isotropic electron distribution models, deciding on a model with two kappa distributions. We adjust our isotropic model to the flux composite spectra with a least square method to produce three sets of fluid parameters (density, temperature, spectral index) per electron population. The radial profiles are then analyzed, revealing a relevant boundary at 9 R-s in both thermal and suprathermal electron populations. Observed discontinuities in the moment profiles (sudden drop-off in cold density profile outside 9 R-s, hot electrons drop-off inside 9 R-s) coincide with the known outer edge of Saturn's neutral OH cloud. Farther out, thermal electrons disappear completely beyond 15 R-s while suprathermal electrons are still observed in the middle and outer magnetosphere.