Global Maps of Energetic Ions in Saturn's Magnetosphere

Abstract

The Magnetospheric Imaging Instrument/Charge Energy Mass Spectrometer onboard the Cassini spacecraft measured suprathermal H⁺ and water group (W⁺) ions during the 13-year mission from 2004 to 2017. This paper generates statistical maps of pulse height analyzed fluxes for 3.24-3.34 keV H⁺ and 8.79-9.05 keV W⁺ from the start of day 2 in 2005 through the end of day 366 in 2016. The fluxes of both H⁺ and W⁺ are more intense on the nightside than on the dayside. The dayside magnetodisk thickness of H⁺ is greater than on the nightside, while the day and night thicknesses are comparable for W⁺. On both dayside and nightside, fluxes concentrate near the equator and also along the magnetic field lines near and just outside the Rhea L shell (L ≈ 8.75R_S, 1R_S = 60,268 km). The flux peaks map to the ionosphere well equatorward of the aurora. This intensity peak is near the boundary between where interchange events occur and where tail collapse (reconnection) events occur. The intensity variations can be represented by lognormal distributions in range r for each latitude λ, and the lognormal coefficients can then be fitted in latitude, thus allowing construction of simple models of the H⁺ and W⁺ spatial distributions. The models can then be used to estimate the fluxes of low energy suprathermal ions from 5 to 21R_S on the dayside and 5 to 30R_S on the nightside in both r and λ, or alternately in ρ and z.