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- Title
Constraining the Temporal Variability of Neutral Winds in Saturn's Low‐Latitude Ionosphere Using Magnetic Field Measurements.
- Authors
Agiwal, Omakshi; Cao, H.; Cowley, S. W. H.; Dougherty, M. K.; . Hunt, G. J; Müller‐Wodarg, I.; Achilleos, N.
- 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. Plain Language Summary: The final set of magnetic field measurements from the Cassini Mission at Saturn showed electrical currents flowing between the northern and southern hemispheres along magnetic field lines close to the equator, which changed on week‐long timescales. This observation was unexpected, and recent studies have shown that these currents are most likely driven by different wind speeds in Saturn's upper atmosphere at the northern and southern end of the magnetic field lines. Presently, we do not have measurements of the winds near the equator in Saturn's upper atmosphere. This study presents a new approach by using magnetic field measurements of the electrical currents to figure out how the winds vary in latitude, longitude, and time. We find that the winds close to the equator may be variable by up to 1,260 km/h, and discuss ways in which our results could be developed to get a more detailed understanding of changes in Saturn's upper atmosphere. Key Points: Magnetic field data reveal low‐latitude field‐aligned currents at Saturn and are used to infer variability of neutral flows in ionosphereWe test many modeled wind profiles and find the peak neutral flows are likely variable by ∼350 m/s near the equator over a 5 months intervalWe present empirically inferred timeseries of inter‐hemispheric angular velocity shears in neutral flows for the low‐latitude ionosphere
- Subjects
CASSINI (Spacecraft); PLANETARY orbits; SATURN exploration; RINGS of Saturn; INTERIOR of Saturn
- Publication
Journal of Geophysical Research. Planets, 2021, Vol 126, Issue 2, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2020JE006578