Energetic particle signatures above Saturn’s aurorae

Bader, Alexander and Badman, Sarah and Ray, Licia C and Paranicas, C. and Lorch, Chris and Clark, George and Andre, Mats and Mitchell, Donald G. and Constable, David A. and Kinrade, Joe and Hunt, Gregory and Pryor, W. R. (2020) Energetic particle signatures above Saturn’s aurorae. Journal of Geophysical Research: Space Physics, 125 (1): e2019JA027. ISSN 2169-9402

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Abstract

Near the end of its mission, NASA's Cassini spacecraft performed several low‐altitude passes across Saturn's auroral region. We present auroral imagery and various coincident particle and field measurements of two such passes, providing important information about the structure and dynamics of Saturn's auroral acceleration region. In upward field‐aligned current regions, upward proton beams are observed to reach energies of several tens of keV; the associated precipitating electron populations are found to have mean energies of about 10 keV. With no significant wave activity being apparent, these findings indicate strong parallel potentials responsible for auroral acceleration; about 100 times stronger than at Earth. This is further supported by observations of proton conics in downward field‐aligned current regions above the acceleration region, which feature a lower energy cutoff above ~50 keV ‐ indicating energetic proton populations trapped by strong parallel potentials while being transversely energized until they can overcome the trapping potential, likely through wave‐particle interactions. A spacecraft pass through a downward current region at an altitude near the acceleration region reveals plasma wave features which may be driving the transverse proton acceleration generating the conics. Overall, the signatures observed resemble those related to the terrestrial and Jovian aurorae, the particle energies and potentials at Saturn appearing to be significantly higher than at Earth and comparable to those at Jupiter.