Midnight flash model of energetic neutral atom periodicities at Saturn

Abstract

The Ion Neutral Camera on the Cassini spacecraft made images of energetic H atoms (25–55 keV) over a 3 day span in 2017. The images were projected onto the equatorial plane of Saturn, and a keogram was made by interpolating the projections in local time at 9 RS (1 RS = 60268 km). The keogram intensities show strong periodicities near the 10.79 h period of Saturn's energetic particles and exhibit a slope commensurate with corotation at that period. These periodic fluxes intensify near midnight but are weaker near noon. A “midnight flash” model can explain this behavior in terms of a searchlight rotating at 10.79 h that intensifies in the midnight sector. The model can also describe similar activity in Saturn's kilometric radiation and magnetic fields, although the “flash” must be shifted to the dawn‐to‐noon sector.

Energetic Electron Periodicities During the Cassini Grand Finale

“…The small Doppler‐related peaks recognizable in the periodograms strongly suggest a rotating phenomenon. Indeed, a simple rotational model such as the midnight‐flash model (Carbary & Mitchell, ) gives a surprisingly accurate description of the periodicity when applied using the actual spacecraft trajectory.…”

“…This model is basically a rotating searchlight pattern that intensifies once per rotation in the midnight sector. Such a model has accurately reproduced periodicities observed in the energetic neutral atoms (Carbary & Mitchell, 2017). The model assumes an exponential falloff of particle fluxes in the radial and z dimensions, a sinusoidal modulation of ωt (where ω = 2π/10.79 h and t is time in hours), and a sinusoidal modulation in local time ϕ (where ϕ = local time of observer).…”

“…These contaminated data have been removed prior to processing, but the remaining observations have not been filtered in any way. This data processing is similar to that used before in previous studies of charged particle periodicities at Saturn (e.g., Carbary & Mitchell, 2017;Carbary, Mitchell, Krimigis, Hamilton, et al, 2007;Carbary et al, 2009Carbary et al, , 2011.…”

“…To reveal the periodicities, the differential fluxes were subjected to a Lomb-Scargle periodogram analysis. This method has been used reliably in the past to expose Saturn's radio and particle periodicities (e.g., Carbary & Mitchell, 2017;Lamy, 2011;Ye et al, 2016). The periodogram window is here limited to 6-14 h to capture the so-called "planetary period" of Saturn.…”

The Dynamics of Saturn's Main Aurorae

“…The quiet auroral oval is overlaid with repeated powerful auroral plasma injection events (Mitchell et al, ) at Saturn's dawnside, which almost never rotate past noon as the perturbed source population's free energy is gradually deposited in Saturn's atmosphere, generating aurorae. The related rotating injected hot plasma populations seen in energetic neutral atom images do not stall at noon but continue rotating near rigidly with diminishing intensity back into the nightside sector where they appear to be reenergized with every pass (Carbary & Mitchell, ; Mitchell et al, ). All injections commence near dawn, indicating nightside reconnection and the consequent magnetic dipolarization (Yao et al, ) as a likely cause (Radioti et al, )‐ considering the significant bendback of the magnetic field at dawn, this LT region maps well into Saturn's nightside.…”