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.

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© 2021. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Issue Online: 18 February 2021; Version of Record online: 18 February 2021; Accepted manuscript online: 13 January 2021; Manuscript accepted: 23 December 2020; Manuscript revised: 21 December 2020; Manuscript received: 16 June 2020. Omakshi Agiwal is supported by STFC grant ST/R504816/1. G. J. Hunt is supported by STFC grant ST/N000692/1. S. W. H. Cowley is supported by STFC grant ST/N000749/1. M. K. Dougherty is funded by Royal Society Research Professorship RP140004. N. Achilleos is supported by STFC grant ST/S000240/1. H. Cao is supported by NASA Jet Propulsion Laboratory (JPL) contract 1579625. H. Cao also acknowledges Royal Society Grant RP/EA/180014 to enable an academic visit to Imperial College London. We would like to thank A. Medvedev for their contribution to the STIM model, and O. Shebanits for useful discussions. This work also benefited from discussions at the "New View of Ring‐Planet Interactions From Cassini's Grand Finale" team meeting, hosted by the International Space Science Institute. Data Availability Statement: Magnetometer data supporting this work are publicly available on the Imperial College London MAGDA server (https://magda.imperial.ac.uk/) and the NASA Planetary Data System at the Jet Propulsion Laboratory (https://pds.jpl.nasa.gov/). Data required to reproduce the results from this study are available from Agiwal (2020).

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Published - 2020JE006578.pdf

Supplemental Material - 2020je006578-sup-0003-figure_si-s01.pdf

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