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Discovery of Atmospheric-Wind-Driven Electric Currents in Saturn's Magnetosphere in the Gap Between Saturn and its Rings

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journal contribution
posted on 2019-07-25, 14:09 authored by KK Khurana, MK Dougherty, G Provan, GJ Hunt, MG Kivelson, SWH Cowley, DJ Southwood, CT Russell
Magnetic field observations obtained by the Cassini spacecraft as it traversed regions inside of Saturn's D ring packed a genuine surprise. The azimuthal component of the magnetic field recorded a consistent positive perturbation with a strength of 15–25 nT near closest approach. The closest approaches were near the equatorial plane of Saturn and were distributed narrowly around local noon and brought the spacecraft to within 2,550 km of Saturn's cloud tops. Modeling of this perturbation shows that it is not of internal origin but is produced by external currents that couple the low-latitude northern ionosphere to the low-latitude southern ionosphere. The azimuthal perturbations diminish at higher latitudes on field lines that connect to Saturn's icy rings. The sense of the current system suggests that the southern feet of the field lines in the ionosphere leads their northern counterparts. We show that the observed field perturbations are consistent with a field-aligned current whose strength is ~1 MA/radian, that is, comparable in strength to the planetary-period-oscillation-related current systems observed in the auroral zone. We show that the Lorentz force in the ionosphere extracts momentum from the faster moving low-latitude zonal belt and delivers it to the northern ionosphere. We further show that the electric current is generated when the two ends of a field line are embedded in zonal flows with differing wind speeds in the low-latitude thermosphere. The wind-generated currents dissipate 2 × 1011W of thermal power, similar to the input from the solar extreme ultraviolet flux in this region.

History

Citation

Geophysical Research Letters, 2018, 45 (19), pp. 10,068-10,074

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

  • VoR (Version of Record)

Published in

Geophysical Research Letters

Publisher

American Geophysical Union (AGU), Wiley

issn

0094-8276

eissn

1944-8007

Acceptance date

2018-07-04

Copyright date

2018

Available date

2019-07-25

Publisher version

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078256

Notes

The data used in this work are readily available from the Planetary Data System.

Language

en

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