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Plasma in Saturn's nightside magnetosphere and the implications for global circulation

Plasma in Saturn's nightside magnetosphere and the implications for global circulation
Plasma in Saturn's nightside magnetosphere and the implications for global circulation

We present a bulk ion flow map from the nightside, equatorial region of Saturn's magnetosphere derived from the Cassini CAPS ion mass spectrometer data. The map clearly demonstrates the dominance of corotation flow over radial flow and suggests that the flux tubes sampled are still closed and attached to the planet up to distances of 50RS. The plasma characteristics in the near-midnight region are described and indicate a transition between the region of the magnetosphere containing plasma on closed drift paths and that containing flux tubes which may not complete a full rotation around the planet. Data from the electron spectrometer reveal two plasma states of high and low density. These are attributed either to the sampling of mass-loaded and depleted flux tubes, respectively, or to the latitudinal structure of the plasma sheet. Depleted, returning flux tubes are not, in general, directly observed in the ions, although the electron observations suggest that such a process must take place in order to produce the low-density population. Flux-tube content is conserved below a limit defined by the mass-loading and magnetic field strength and indicates that the flux tubes sampled may survive their passage through the tail. The conditions for mass-release are evaluated using measured densities, angular velocities and magnetic field strength. The results suggest that for the relatively dense ion populations detectable by the ion mass spectrometer (IMS), the condition for flux-tube breakage has not yet been exceeded. However, the low-density regimes observed in the electron data suggest that loaded flux tubes at greater distances do exceed the threshold for mass-loss and subsequently return to the inner magnetosphere significantly depleted of plasma.

Ions, Magnetospheric, Magnetotail, Planetary wind, Saturn
0032-0633
1714-1722
McAndrews, H.J.
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Thomsen, M.F.
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Arridge, C.S.
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Jackman, C.M.
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Wilson, R.J.
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Henderson, M. G.
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Tokar, R.L.
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Khurana, K.K.
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Sittler, E.C.
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Coates, A.J.
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Dougherty, M.K.
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McAndrews, H.J.
0e419a05-f21c-413e-9913-462fc341171b
Thomsen, M.F.
1b7a2912-f9ac-47c0-b4a1-a9b7f435e306
Arridge, C.S.
2efb7acf-be5b-41b4-bade-8b4ff0d78236
Jackman, C.M.
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Wilson, R.J.
9c148912-45a3-44b4-8f1c-13b56c2df578
Henderson, M. G.
8b14c275-d7ff-47f7-ad8f-7189b6d2e152
Tokar, R.L.
8213ad4b-5b3b-46b1-90ad-334b61c2d79c
Khurana, K.K.
fdd7f0dc-99b0-4e66-a0e2-adbe022cc3dd
Sittler, E.C.
25061066-c935-4da0-beaf-01f6e9208446
Coates, A.J.
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Dougherty, M.K.
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McAndrews, H.J., Thomsen, M.F., Arridge, C.S., Jackman, C.M., Wilson, R.J., Henderson, M. G., Tokar, R.L., Khurana, K.K., Sittler, E.C., Coates, A.J. and Dougherty, M.K. (2009) Plasma in Saturn's nightside magnetosphere and the implications for global circulation. Planetary and Space Science, 57 (14-15), 1714-1722. (doi:10.1016/j.pss.2009.03.003).

Record type: Article

Abstract

We present a bulk ion flow map from the nightside, equatorial region of Saturn's magnetosphere derived from the Cassini CAPS ion mass spectrometer data. The map clearly demonstrates the dominance of corotation flow over radial flow and suggests that the flux tubes sampled are still closed and attached to the planet up to distances of 50RS. The plasma characteristics in the near-midnight region are described and indicate a transition between the region of the magnetosphere containing plasma on closed drift paths and that containing flux tubes which may not complete a full rotation around the planet. Data from the electron spectrometer reveal two plasma states of high and low density. These are attributed either to the sampling of mass-loaded and depleted flux tubes, respectively, or to the latitudinal structure of the plasma sheet. Depleted, returning flux tubes are not, in general, directly observed in the ions, although the electron observations suggest that such a process must take place in order to produce the low-density population. Flux-tube content is conserved below a limit defined by the mass-loading and magnetic field strength and indicates that the flux tubes sampled may survive their passage through the tail. The conditions for mass-release are evaluated using measured densities, angular velocities and magnetic field strength. The results suggest that for the relatively dense ion populations detectable by the ion mass spectrometer (IMS), the condition for flux-tube breakage has not yet been exceeded. However, the low-density regimes observed in the electron data suggest that loaded flux tubes at greater distances do exceed the threshold for mass-loss and subsequently return to the inner magnetosphere significantly depleted of plasma.

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More information

Published date: 1 December 2009
Keywords: Ions, Magnetospheric, Magnetotail, Planetary wind, Saturn

Identifiers

Local EPrints ID: 437335
URI: http://eprints.soton.ac.uk/id/eprint/437335
ISSN: 0032-0633
PURE UUID: fd1714f3-8327-4d6b-a6ba-9cae494905c2
ORCID for C.M. Jackman: ORCID iD orcid.org/0000-0003-0635-7361

Catalogue record

Date deposited: 24 Jan 2020 17:31
Last modified: 10 Nov 2021 03:35

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Contributors

Author: H.J. McAndrews
Author: M.F. Thomsen
Author: C.S. Arridge
Author: C.M. Jackman ORCID iD
Author: R.J. Wilson
Author: M. G. Henderson
Author: R.L. Tokar
Author: K.K. Khurana
Author: E.C. Sittler
Author: A.J. Coates
Author: M.K. Dougherty

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