Global Maps of Energetic Ions in Saturn's Magnetosphere

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 approximate to 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 lambda, 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 similar to 5 to similar to 21R(S) on the dayside and similar to 5 to similar to 30R(S) on the nightside in both r and lambda, or alternately in rho and z. Plain Language Summary The spatial distribution of ions in a magnetosphere is of fundamental interest to space scientists. This paper describes the distribution of protons (H+) and water group ions (mostly, O+) in the magnetosphere of Saturn as derived from 12 years of observations by the MIMI/CHEMS instrument on the Cassini spacecraft. Definite patterns in these flux maps appear. For instance, the fluxes of these ions follow magnetic field lines that cross the equator just outside the orbit of the moon Rhea. A model is constructed that has wide applicability to other investigations of Saturn's magnetosphere.