Ion Phase Space Density Profiles Within the Magnetosphere of Saturn: Investigating the Source of Suprathermal Ions

Abstract

Phase space density (PSD) profiles at fixed values of the first two adiabatic invariants ($\mu$; magnetic moment and $k$; longitudinal invariant) versus L shell provide a quantitative way to investigate the sources, losses, and radial transport of plasma and energetic charged particles within a planetary magnetosphere. PSD profiles have been obtained from the Magnetosphere Imaging Investigation/Charge-Energy-Mass Spectrometer (MIMI/CHEMS) for H⁺, W⁺, O⁺⁺, H₂⁺, He⁺, and He⁺⁺ for the period Cassini was in orbit around Saturn (July 2004 until September 2017). Beyond the expected variations for all species due to the suprathermal spectral slopes, the PSD profiles also show significantly more loss for lower $k$ (i.e., more equatorially mirroring or “trapped”) populations. This dependency of $k$ for loss is consistent with higher loss rates of trapped populations in the inner magnetosphere (<10 R_S) due to neutral densities being more equatorially confined. The PSD radial profiles of all species, regardless of solar wind-origin (e.g., H⁺, He⁺, He⁺⁺) or magnetospheric-origin (e.g., H⁺, W⁺, H₂⁺, and O⁺⁺), indicate that the outer magnetosphere is the predominant region of non-adiabatically accelerated lower energy plasma that may be a source of the suprathermal ions. Mission-averaged PSD profiles are also used along with the computed PSD profile near an interchange injection event to estimate the original L shell of injected H⁺ and W⁺ ions. For this injection event, the estimated source of both the H⁺ and W⁺ ions was between 1 and 2 Saturn radii (R_S) away, consistent with previous studies of interchange injection events at Saturn.