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Examining local time variations in the gains and losses of open magnetic flux during substorms

Examining local time variations in the gains and losses of open magnetic flux during substorms
Examining local time variations in the gains and losses of open magnetic flux during substorms

The open magnetic flux content of the magnetosphere varies during substorms as a result of dayside and nightside reconnection. The open flux can be calculated from the area of the polar cap, delineated by the open-closed field line boundary (OCB). This study presents a superposed epoch analysis of the location of the OCB and the change in the magnetic flux content in individual nightside MLT sectors during substorm growth, expansion, and recovery phases. Far ultraviolet (FUV) observations from the IMAGE satellite are used to derive a proxy of the OCB location. In the hour prior to substorm onset, the total nightside flux content increases by up to 0.12 GWb on average, resulting in an equatorward expansion of the OCB. Following substorm onset, the OCB contracts toward the pole as the open magnetic flux content decreases by up to 0.14 GWb on average, but the rate of decrease of the total nightside open flux content differs by 5–66% between the three IMAGE far ultraviolet instruments. The OCB does not contract poleward uniformly in all nightside magnetic local time (MLT) sectors after substorm onset. Close to the substorm onset MLT sector, the OCB contracts immediately following substorm onset; however, the OCB in more dawnward and duskward MLT sectors continues to expand equatorward for up to 120 minutes after substorm onset. Despite the continued increase in flux in these sectors after substorm onset, the total nightside flux content decreases immediately at substorm onset, indicating that the nightside reconnection rate exceeds the dayside rate following substorm onset.

auroral oval, IMAGE FUV, open-closed field line boundary, substorms, superposed epoch analysis, total flux content
2169-9380
Mooney, M. K.
e40bb953-bbc9-4047-a8ca-9b90d3419357
Forsyth, C.
f3d464cf-81c5-4647-b85a-bc3634bdf076
Rae, I. J.
25677763-0501-4b69-859e-fab930260c69
Chisham, G.
25a4afbd-b568-49c4-a881-206da96f8425
Coxon, J. C.
566baea5-6a30-4855-bde3-a09c115efde4
Marsh, M. S.
e5fad144-c28e-4b52-9d48-95b28489e3b6
Jackson, D. R.
842fd157-1d2d-4d2f-a3b1-81777b0ea557
Bingham, S.
1e0cc4a8-d340-4794-bf19-72f35b69beb6
Hubert, B.
0472a694-50b8-4bed-8f30-8c5fcbbd58dc
Mooney, M. K.
e40bb953-bbc9-4047-a8ca-9b90d3419357
Forsyth, C.
f3d464cf-81c5-4647-b85a-bc3634bdf076
Rae, I. J.
25677763-0501-4b69-859e-fab930260c69
Chisham, G.
25a4afbd-b568-49c4-a881-206da96f8425
Coxon, J. C.
566baea5-6a30-4855-bde3-a09c115efde4
Marsh, M. S.
e5fad144-c28e-4b52-9d48-95b28489e3b6
Jackson, D. R.
842fd157-1d2d-4d2f-a3b1-81777b0ea557
Bingham, S.
1e0cc4a8-d340-4794-bf19-72f35b69beb6
Hubert, B.
0472a694-50b8-4bed-8f30-8c5fcbbd58dc

Mooney, M. K., Forsyth, C., Rae, I. J., Chisham, G., Coxon, J. C., Marsh, M. S., Jackson, D. R., Bingham, S. and Hubert, B. (2020) Examining local time variations in the gains and losses of open magnetic flux during substorms. Journal of Geophysical Research: Space Physics, 125 (4), [e2019JA027369]. (doi:10.1029/2019JA027369).

Record type: Article

Abstract

The open magnetic flux content of the magnetosphere varies during substorms as a result of dayside and nightside reconnection. The open flux can be calculated from the area of the polar cap, delineated by the open-closed field line boundary (OCB). This study presents a superposed epoch analysis of the location of the OCB and the change in the magnetic flux content in individual nightside MLT sectors during substorm growth, expansion, and recovery phases. Far ultraviolet (FUV) observations from the IMAGE satellite are used to derive a proxy of the OCB location. In the hour prior to substorm onset, the total nightside flux content increases by up to 0.12 GWb on average, resulting in an equatorward expansion of the OCB. Following substorm onset, the OCB contracts toward the pole as the open magnetic flux content decreases by up to 0.14 GWb on average, but the rate of decrease of the total nightside open flux content differs by 5–66% between the three IMAGE far ultraviolet instruments. The OCB does not contract poleward uniformly in all nightside magnetic local time (MLT) sectors after substorm onset. Close to the substorm onset MLT sector, the OCB contracts immediately following substorm onset; however, the OCB in more dawnward and duskward MLT sectors continues to expand equatorward for up to 120 minutes after substorm onset. Despite the continued increase in flux in these sectors after substorm onset, the total nightside flux content decreases immediately at substorm onset, indicating that the nightside reconnection rate exceeds the dayside rate following substorm onset.

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More information

Accepted/In Press date: 25 February 2020
e-pub ahead of print date: 28 February 2020
Published date: 1 April 2020
Keywords: auroral oval, IMAGE FUV, open-closed field line boundary, substorms, superposed epoch analysis, total flux content

Identifiers

Local EPrints ID: 441024
URI: http://eprints.soton.ac.uk/id/eprint/441024
ISSN: 2169-9380
PURE UUID: 1b111d3e-169d-483e-8b60-5b857b08d869
ORCID for J. C. Coxon: ORCID iD orcid.org/0000-0002-0166-6854

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Date deposited: 27 May 2020 16:55
Last modified: 18 Feb 2021 17:24

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Contributors

Author: M. K. Mooney
Author: C. Forsyth
Author: I. J. Rae
Author: G. Chisham
Author: J. C. Coxon ORCID iD
Author: M. S. Marsh
Author: D. R. Jackson
Author: S. Bingham
Author: B. Hubert

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