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Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics

Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics
Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics

We study the interplanetary magnetic field (IMF) data obtained by the Cassini spacecraft during a ∼6.5-month interval when the spacecraft was approaching Saturn at heliocentric distances between ∼8.5 and ∼8.9 AU. It is shown that the structure of the IMF is consistent with that expected to be formed by corotating interaction regions (CIRs) during the declining phase of the solar cycle, with two sectors during each solar rotation, and crossings of the heliospheric current sheet generally embedded within few-day higher-field compression regions, separated by several-day lower-field rarefaction regions. This pattern was disrupted in November 2003, however, by an interval of high activity on the Sun. These data have then been employed to estimate the voltage associated with open flux production at Saturn's magnetopause using an empirical formula adapted from Earth. The results show that the CIR-relaved structuring of the IMF leads to corresponding structuring of the interplanetary interaction with Saturn's magnetosphere and hence also to intervals of very different dynamical behavior. During few-day compression regions where the IMF strength is ∼0.5-2 nT, the average Dungey cycle voltage is estimated to be ∼100 kV, such that the open flux produced over such intervals is ∼30-40 GWb, similar to the typical total amount present in Saturn's magnetosphere. The magnetosphere is thus significantly driven by the solar wind interaction during such intervals. During some rarefaction intervals, on the other hand, the field strength remains ∼0.1 nT or less over several days, implying reconnection voltages of ∼10 kV or less, with negligible production of open flux. The magnetosphere is then expected to enter a quiescent state, dominated by internal processes. Overall, ∼100 GWb of open flux is estimated to be produced during each ∼25-day solar rotation, about 3 times the typical flux contained in the tail, and sufficient to drive three to five substorms. We point out, however, that CIR-related variations in solar wind dynamic pressure will also occur in synchronism with the field variations, which may also play a role in modulating the open flux in the system, thus reinforcing the synchronization of the pattern of growth and decay of open flux to the CIR pattern. Estimates of open flux production associated with the period of strong solar activity indicate that major magnetospheric dynamics were excited by reconnection- mediated solar wind interaction during this interval.

Cassini space mission, Interplanetary magnetic field, Reconnection voltage, Saturn's magnetospheric dynamics, Solar wind-magnetosphere coupling, Substorms
2169-9380
1-19
Jackman, C.M.
9bc3456c-b254-48f1-ade0-912c5b8b4529
Achilleos, N.
620cd146-20a4-4a84-8fd9-2a3e5d95aa2a
Bunce, E.J.
208e1177-a92f-47a6-878d-ef27ed03f811
Cowley, S.W.H.
e8174910-ddcd-4a23-9d7c-0e1117def06c
Dougherty, M.K.
aa297043-7e86-4501-952b-a6363edaaba8
Jones, G.H.
7bb876d5-718d-4948-b21f-de09ec1eeeb7
Milan, S.E.
b6dd5367-b0c4-4f30-953a-f2d13078d656
Smith, E.J.
504828bd-88c4-43c7-959f-3f40315efc59
Jackman, C.M.
9bc3456c-b254-48f1-ade0-912c5b8b4529
Achilleos, N.
620cd146-20a4-4a84-8fd9-2a3e5d95aa2a
Bunce, E.J.
208e1177-a92f-47a6-878d-ef27ed03f811
Cowley, S.W.H.
e8174910-ddcd-4a23-9d7c-0e1117def06c
Dougherty, M.K.
aa297043-7e86-4501-952b-a6363edaaba8
Jones, G.H.
7bb876d5-718d-4948-b21f-de09ec1eeeb7
Milan, S.E.
b6dd5367-b0c4-4f30-953a-f2d13078d656
Smith, E.J.
504828bd-88c4-43c7-959f-3f40315efc59

Jackman, C.M., Achilleos, N., Bunce, E.J., Cowley, S.W.H., Dougherty, M.K., Jones, G.H., Milan, S.E. and Smith, E.J. (2004) Interplanetary magnetic field at ∼9 AU during the declining phase of the solar cycle and its implications for Saturn's magnetospheric dynamics. Journal of Geophysical Research: Space Physics, 109 (A11), 1-19, [A11203]. (doi:10.1029/2004JA010614).

Record type: Article

Abstract

We study the interplanetary magnetic field (IMF) data obtained by the Cassini spacecraft during a ∼6.5-month interval when the spacecraft was approaching Saturn at heliocentric distances between ∼8.5 and ∼8.9 AU. It is shown that the structure of the IMF is consistent with that expected to be formed by corotating interaction regions (CIRs) during the declining phase of the solar cycle, with two sectors during each solar rotation, and crossings of the heliospheric current sheet generally embedded within few-day higher-field compression regions, separated by several-day lower-field rarefaction regions. This pattern was disrupted in November 2003, however, by an interval of high activity on the Sun. These data have then been employed to estimate the voltage associated with open flux production at Saturn's magnetopause using an empirical formula adapted from Earth. The results show that the CIR-relaved structuring of the IMF leads to corresponding structuring of the interplanetary interaction with Saturn's magnetosphere and hence also to intervals of very different dynamical behavior. During few-day compression regions where the IMF strength is ∼0.5-2 nT, the average Dungey cycle voltage is estimated to be ∼100 kV, such that the open flux produced over such intervals is ∼30-40 GWb, similar to the typical total amount present in Saturn's magnetosphere. The magnetosphere is thus significantly driven by the solar wind interaction during such intervals. During some rarefaction intervals, on the other hand, the field strength remains ∼0.1 nT or less over several days, implying reconnection voltages of ∼10 kV or less, with negligible production of open flux. The magnetosphere is then expected to enter a quiescent state, dominated by internal processes. Overall, ∼100 GWb of open flux is estimated to be produced during each ∼25-day solar rotation, about 3 times the typical flux contained in the tail, and sufficient to drive three to five substorms. We point out, however, that CIR-related variations in solar wind dynamic pressure will also occur in synchronism with the field variations, which may also play a role in modulating the open flux in the system, thus reinforcing the synchronization of the pattern of growth and decay of open flux to the CIR pattern. Estimates of open flux production associated with the period of strong solar activity indicate that major magnetospheric dynamics were excited by reconnection- mediated solar wind interaction during this interval.

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More information

Published date: 1 January 2004
Keywords: Cassini space mission, Interplanetary magnetic field, Reconnection voltage, Saturn's magnetospheric dynamics, Solar wind-magnetosphere coupling, Substorms

Identifiers

Local EPrints ID: 437116
URI: http://eprints.soton.ac.uk/id/eprint/437116
ISSN: 2169-9380
PURE UUID: 9e072ca9-a2b1-436f-a588-a5d2dd015738
ORCID for C.M. Jackman: ORCID iD orcid.org/0000-0003-0635-7361

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Date deposited: 17 Jan 2020 17:34
Last modified: 18 Feb 2021 17:23

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Contributors

Author: C.M. Jackman ORCID iD
Author: N. Achilleos
Author: E.J. Bunce
Author: S.W.H. Cowley
Author: M.K. Dougherty
Author: G.H. Jones
Author: S.E. Milan
Author: E.J. Smith

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