Constraining the Temporal Variability of Neutral Winds in Saturn's Low-Latitude Ionosphere Using Magnetic Field Measurements

Abstract

The Cassini spacecraft completed 22 orbits around Saturn known as the “Grand Finale” over a 5 months interval, during which time the spacecraft traversed the previously unexplored region between Saturn and its equatorial rings near periapsis. The magnetic field observations reveal the presence of temporally variable low-latitude field-aligned currents which are thought to be driven by velocity shears in the neutral zonal winds at magnetically conjugate thermospheric latitudes. We consider atmospheric waves as a plausible driver of temporal variability in the low-latitude thermosphere, and empirically constrain the region in which they perturb the zonal flows to be between ±25° latitude. By investigating an extensive range of hypothetical wind profiles, we present and analyze a timeseries of the modeled velocity shears in thermospheric zonal flows, with direct comparisons to empirically inferred angular velocity shears from the Bϕ observations. We determine the maximum temporal variability in the peak neutral zonal winds over the Grand Finale interval to be ∼350 m/s assuming steady-state ionospheric Pedersen conductances. We further show that the ionospheric currents measured must be in steady-state on ∼10 min timescales, and axisymmetric over ∼2 h of local time in the near-equatorial ionosphere. Our study illustrates the potential to use of magnetospheric datasets to constrain atmospheric variability in the thermosphere region.