Abstract
AbstractNear the end of its mission, NASA's Cassini spacecraft performed several low‐altitude passes across Saturn's auroral region. We present ultraviolet 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.
Highlights
In the final year of its mission, NASA's Cassini spacecraft performed a set of orbits bringing it closer to Saturn than ever before
The two auroral passes presented above involve a number of different processes all occurring in the vicinity of auroral field lines
No proton features could be observed as the main upward current region was traversed, but a weak proton beam equatorward of the auroral oval increased in intensity with decreasing spacecraft altitude
Summary
In the final year of its mission, NASA's Cassini spacecraft performed a set of orbits bringing it closer to Saturn than ever before This presented a unique opportunity to combine remote sensing of Saturn's aurorae in unprecedented spatial resolution with in situ measurements of particles and fields advancing our understanding of the auroral acceleration process which has so far been investigated mostly at Earth and Jupiter. More evidence for wave-particle interactions similar to terrestrial processes has been found by Menietti et al (2011), who investigated the relation between ion cyclotron harmonics and electron beams observed at the same time Their modeling suggests that this process can produce significant ion heating as previously investigated for Earth's auroral region (Singh et al, 1981).
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