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A localized and surprising source of energetic particles in the Uranian magnetosphere near Ariel

Presentation #324.08 in the session Giant Planet Magnetospheres, Ionospheres and Aurorae (Poster + Lightning Talk)

Published onOct 23, 2023
A localized and surprising source of energetic particles in the Uranian magnetosphere near Ariel

A fresh survey of energetic particle observations from the Low Energy Charged Particle (LECP) instrument on Voyager 2 revealed a previously underappreciated signature. Specifically, LECP observed a significant (several orders of magnitude) discrepancy between the intensity of energetic particles (both ions and electrons) observed during the outbound leg of the Uranus flyby encounter in the region between Miranda and Ariel compared to the inbound leg. Of particular interest are the pitch angle distributions (PADs) measured by LECP, which display extremely steep gradients. Such a steep gradient in pitch angle is difficult to maintain since any waves would act to scatter the particles and isotropize the distribution. Maintaining such a steep PAD would require a significant and relatively constant source of energetic particles, specifically for those at near-90° pitch angles, at rates that can balance or even overcome any loss/scattering processes from waves. To assess whether such an energetic particle source is in fact present in the Uranian system, distributions of the phase space density (PSD) profiles of the ions were investigated. A clear maximum between Miranda and Ariel at L~7 suggests a source of energetic ions in this region. Potential energetic particle sources include particle injections, cosmic ray albedo neutron decay (CRAND), and an active moon (i.e., a candidate ocean world). Both particle injections and CRAND are believed to be unlikely sources. However, an active moon source is potentially plausible. First, the narrow pitch angle source required matches that expected from newly created pickup ions. Other Voyager 2 observations also provide potential additional evidence to support an active moon source. Ultimately, further investigation of this mysterious energetic particle source will require additional observations from the Uranian system, preferably from an orbiter mission that is equipped with instruments to measure the thermal plasma properties and composition, suprathermal (tens to hundreds of keV) ion composition, with both mass and charge-state, and wave activity extending into the ion-cyclotron modes (i.e., EMIC waves).

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