Energetic Particle Signatures Above Saturn's Aurorae

Abstract

Near 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.Plain Language Summary NASA's Cassini spacecraft orbited closer to Saturn than ever before during the last stage of its mission, the "Grand Finale". This allowed the onboard instruments to measure charged particles and plasma waves directly above the auroral region while simultaneously providing high-resolution imagery of the ultraviolet aurorae. Based on observations of highly energetic ions streaming away from the planet in regions of low plasma wave activity, we infer the existence of strong electric fields which act to accelerate electrons down into the atmosphere, driving the bright auroral emissions. Our estimates of the average energy of the precipitating electrons support this finding. Charged ions sometimes seem to be energized by plasma waves above the aurorae before they can escape, but the exact process in which this happens is not fully understood. Most signatures presented here resemble those observed in relation to Earth's aurorae, suggesting that the mechanisms acting at both planets are quite similar although Saturn's acceleration mechanism is significantly stronger.

Jupiter's Low‐Altitude Auroral Zones: Fields, Particles, Plasma Waves, and Density Depletions

“…This is usually an indication of strong damping via cyclotron resonance where the wave energy is transferred to the corresponding ions. This characteristic is consistent with ion cyclotron waves and their observation in the presence of upward energetic ions and downward energetic electron beams draw a strong analogy to both Earth's and Saturn's upward current regions where the correlation has been observed (e.g., Bader et al, 2020;Cattell et al, 1988;McFadden et al, 1998; , 2009). Ion cyclotron waves in the auroral regions have been observed as both electrostatic (EIC) and electromagnetic (EMIC) modes.…”

A Complete Data Set of Equatorial Projections of Saturn's Energetic Neutral Atom Emissions Observed by Cassini‐INCA

“…The high ToF resolution data in Figure 4i shows the measured "ENA" intensity in different energy ranges, and it is clear that ion signatures in ENA imagery correspond to sharp flux increases by sometimes more than an order of magnitude especially in the higher energy bins. The origin of these ion signatures is discussed in, for example, Mitchell, Kurth, et al (2009), Badman et al (2012), andBader et al (2020). This property is used to identify ion contamination events throughout the entire Cassini mission.…”

“…INCA had an instantaneous field‐of‐view (FOV) of 120° × 90° and an angular resolution of up to 64 × 64 pixels, that is, (120/64)° × (90/64)°. When operated in ion mode, INCA was hence able to resolve a significant part of the local ion pitch angle distribution, revealing for example field‐aligned ion beams and conics on auroral field lines (Bader et al., 2020; Mitchell, Kurth, et al., 2009).…”

“…ENA emissions can often be mapped directly to ultraviolet auroral emissions through field‐aligned current systems connecting Saturn's polar auroral ionospheres to the equatorial ring current plasma (Kinrade, Badman, et al., 2020; Mitchell, Krimigis, et al., 2009). While observations of ion and electron beams on auroral field lines have been studied occasionally (Bader et al., 2020; Mitchell, Kurth, et al., 2009), holistic investigations of the relation between ENAs and the aurorae are needed to gain a better understanding of the physical process linking the two.…”

The Statistical Morphology of Saturn’s Equatorial Energetic Neutral Atom Emission

“…Toward these goals, INCA was capable of observing either ions or ENAs in keV energy ranges and determining their mass, energy, and direction of motion. Its large field of view covering 120 ° × 90 ° allowed it to measure significant parts of the ion pitch angle distribution when operated in ion mode (Bader, Badman, et al., 2020; Mitchell, Kurth, et al., 2009), or to perform global observations of ENAs in Saturn’s magnetosphere when operated in neutral mode.…”