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Seasonal Dependence of the Magnetospheric Drag Torque on Saturn's Northern and Southern Polar Thermospheres and its Relation to the Periods of Planetary Period Oscillations

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posted on 2022-08-31, 13:15 authored by S.W.H. Cowley, N. Achilleos, T.J. Bradley, G. Provan, E.J. Bunce, G.J. Hunt
We calculate the magnetospheric drag torques on Saturn's northern and southern polar thermospheres during late southern summer in 2008 and northern spring in 2012–2013 using previously derived profiles of ionospheric meridional coupling currents determined from high-latitude Cassini magnetic field data. We show that the drag torques in the “winter” and “summer” auroral regions are near equal at ~2.3 × 1016 N m, contrary to the recent discussion of Brooks et al. (2019, https://doi.org/10.1002/2019JA026870) who suggest that significant seasonal differences should occur in these regions. Instead, seasonally dependent torques occur in the adjacent polar open field regions, where the “winter” and “summer” torques are ~0.3 × 1016 and ~1.8 × 1016 N m, respectively. We derive a simple rotating disc model of the polar thermosphere and estimate the speed of the poleward flow from midlatitudes required to balance these torques in steady state, finding values of tens of m s−1 consistent with previous numerical modeling. Comparison of the calculated torques with concurrent periods of the northern and southern planetary period oscillations (PPOs) does not suggest a direct connection between these quantities as proposed by Brooks et al., 2019, showing at the least that significant additional factors must be involved. We further note some issues with their scenario for dual modulation of radio emissions, previous observations having shown that the principal oscillatory PPO field-aligned currents that modulate the emissions rotate in the auroral region with periods ~10.7 ± 0.1 hr, propagating through the more slowly rotating ~15–20 hr period outer magnetospheric plasma, with implications for the proposed “atmospheric flywheel” picture.

Funding

STFC. Grant Numbers: ST/N000749/1, ST/S000240/1, ST/N504117/1, ST/N000749/1

History

Citation

Journal of Geophysical Research: Space Physics,125, e2020JA02824

Author affiliation

Department of Physics and Astronomy

Version

  • VoR (Version of Record)

Published in

Journal of Geophysical Research: Space Physics

Volume

125

Issue

9

Publisher

American Geophysical Union

issn

2169-9402

eissn

2169-9402

Acceptance date

2020-08-25

Copyright date

2020

Available date

2022-08-31

Language

English