Discovery of atmospheric-wind-driven electric currents in Saturn's magnetosphere in the gap between Saturn and its rings
File(s)Khurana_et_al-2018-Geophysical_Research_Letters.pdf (1.2 MB)
Published version
Author(s)
Type
Journal Article
Abstract
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.
Date Issued
2018-10-16
Date Acceptance
2018-07-04
Citation
Geophysical Research Letters, 2018, 45 (19), pp.10068-10074
ISSN
0094-8276
Publisher
American Geophysical Union
Start Page
10068
End Page
10074
Journal / Book Title
Geophysical Research Letters
Volume
45
Issue
19
Copyright Statement
©2018. American Geophysical Union. All Rights Reserved.
Sponsor
The Royal Society
Science and Technology Facilities Council (STFC)
Science and Technology Facilities Council (STFC)
Identifier
http://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=PARTNER_APP&SrcAuth=LinksAMR&KeyUT=WOS:000448656800005&DestLinkType=FullRecord&DestApp=ALL_WOS&UsrCustomerID=1ba7043ffcc86c417c072aa74d649202
Grant Number
RP140004
ST/N000692/1
ST/P006922/1
Subjects
Science & Technology
Physical Sciences
Geosciences, Multidisciplinary
Geology
THERMOSPHERE
IONOSPHERE
PERIOD
Publication Status
Published
Date Publish Online
2018-10-03