A Statistical Picture of Pitch Angles of Interchange Events at Saturn and Implications for Energization and Losses

Abstract: We utilize data from the CHEMS instrument to present a statistical review of pitch angles of interchange injections.

Interchange events are a primary mass transport process similar to a Rayleigh-Taylor like instability within ~5–12 Saturn radii (R_s). This instability is from: (1) a flux tube gradient in the plasma population arising from Enceladus located at 4 Rs which outgasses ~250 kg/s of H20 into the magnetosphere and (2) Saturn’s rapid rotation, leading to a co-rotating plasma environment in <11 hours per rotation. This forces dense plasma resulting from the H₂O into less dense plasma (energetic H⁺). The observed transport of energetic H+ toward Saturn is termed interchange injections and can characterize dynamics of the magnetosphere.

We use H+ flux CHEMS to calculate an anisotropy ratio, which compares flux at perpendicular pitch angles to flux at parallel and antiparallel pitch angles. We compare this ratio within interchange events to global distributions. By reviewing pitch angles of interchange events we obtain information about energization (primarily through betatron acceleration), and losses. We build off a survey to identify and classify interchange injections over the mission from high-energy (3–220 keV) ion fluxes. This survey was created through a trained, tested, and automated process; providing events which are statistically classed by severity against the background plasma.

We observe greatly anisotropic interchange events between 7-8 R_s (upwards to >10 times higher perpendicular flux) in the 47–72 keV energy range. Outside of 7-8 R_s we see a decrease of anisotropy to near parity at 12 R_s. We compare energization to expectations of betatron acceleration. Inside of 7–8 Rs we observe depletion of perpendicular flux, which we compare and attribute to expected charge exchange lifetimes for H₂O, O, and H to H⁺.

Azari, A. R., Liemohn, M. W., Jia, X., Allen, R. C., Dewey, R. M., Mitchell, D. G., Paranicas, C., and Sergis, N. “Statistical Picture of Pitch Angles of Interchange Events: Implications for Energization and Loss”, Magnetospheres of the Outer Planets. June, 2019. Sendai, Japan.

My immense gratitude to the local organizing committee for supporting my travel to MOP 2019.