The Generation of Saturn's Aurora at Lower Latitudes by Electrostatic Waves

Abstract

In the present paper we have modeled diffuse auroral emissions, which have been observed at lower latitudes as compared to the main auroral arc at Saturn. It is generally accepted that these emissions are generated by precipitation of hot population of magnetospheric electrons on the closed field lines. We have considered the pitch angle diffusion by electrostatic electron cyclotron harmonic waves as the mechanism to diffuse trapped electrons into the atmospheric loss cone. Electrons are thereby precipitated into the atmosphere and excite the neutral atmospheric constituents producing auroral emissions. Calculation of ultraviolet emission intensities has been performed at two L‐shells 5.35 and 7.0. Observed hot electron distribution functions and observed electrostatic electron cyclotron harmonic wave characteristics have been used in the study. Intensities of Lyman‐alpha emission from excitation of atomic H and Lyman‐alpha produced from dissociative excitation of H2 have been calculated, obtaining the values 20.1 R (1.8 R) and 1.6 kR (0.29 kR), respectively, for both L‐shells 7.0 (5.35).Further, intensities of Lyman and Werner bands of H2 are also calculated. These have the values 7.50 kR (1.43 kR) and 7.52 kR (1.49 kR), respectively, for L‐shells 7.0 (5.35). It is observed that the volume excitation rates of these four excitations peak at an altitude of about 2,000 km. From the range‐energy relation in H2 gas it is estimated that the electrons producing these excitations should have energies less than about 250 eV.

The Morphology of Saturn's Aurorae Observed During the Cassini Grand Finale

“…These outer boundaries map to a radial distance of 6–7 in the magnetic equator plane, corresponding to the inner edge of the region of hot ion/electron plasma as determined in equatorial data (Schippers et al, ; Kellett et al, , ; Carbary et al, ; Carbary, ). The “diffuse” emission observed here and in previous studies is consistent with wave‐driven precipitation from this hot plasma population (Grodent et al, ; Tripathi et al, ), similar to the diffuse outer emission in Jupiter's aurorae (Radioti et al, ).…”

“…Cassini's Ultraviolet Imaging Spectrograph (UVIS) however provided many more observations (visible in, e.g., Radioti et al, ), which will here be exploited to further investigate this signature. It is believed to be caused by hot electrons between 7 and 10 (Schippers et al, ) which may reach the ionosphere through pitch angle scattering by plasma waves (Grodent et al, ; Grodent, ; Tripathi et al, ).…”

Cassini UVIS Detection of Saturn's North Polar Hexagon in the Grand Finale Orbits

“…Grodent et al (2010) described these more diffuse outer auroras in the south polar case as a 7-degree-wide emission centered near planetocentric latitude 67°S attributed to suprathermal (~keV) electrons in the middle magnetosphere. Tripathi et al (2018) calculated that the observed diffuse outer emissions could be caused by pitch angle scattering of softer electrons (less than ~250 eV) by electron cyclotron harmonic waves. Lamy et al (2018) observed diffuse outer auroral emissions in HST images at planetocentric latitudes 65-72°N and presented evidence for field-aligned currents at the time when Cassini crossed this secondary oval in 2017.…”

Excitation of Saturnian ECH Waves Within Remote Plasma Injections: Cassini Observations

“…Via cyclotron resonant interactions, ECH waves can pitch angle scatter plasma sheet electrons into loss cone in the terrestrial magnetosphere, leading to the excitation of diffuse auroral emissions (e.g., Ni et al., 2008, 2011a, 2011b, 2012, 2016, 2017; Horne et al., 2003; Lou et al., 2021, 2022; Meredith et al., 2009; Thorne et al., 2010). Electron scattering by ECH waves is also considered as an important mechanism for producing diffuse aurora at Saturn (Tripathi et al., 2018).…”