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Seasonal and temporal variations of field-aligned currents and ground magnetic deflections during substorms

Seasonal and temporal variations of field-aligned currents and ground magnetic deflections during substorms
Seasonal and temporal variations of field-aligned currents and ground magnetic deflections during substorms

Field-aligned currents (FACs), also known as Birkeland currents, are the agents by which energy and momentum are transferred to the ionosphere from the magnetosphere and solar wind. This coupling is enhanced at substorm onset through the formation of the substorm current wedge. Using FAC data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet technique, we examine the Northern Hemisphere FACs in all local time sectors with respect to substorm onset and subdivided by season. Our results show that while there is a strong seasonal dependence on the underlying FACs, the increase in FACs following substorm onset only varies by 10% with season, with substorms increasing the hemispheric FACs by 420 kA on average. Over an hour prior to substorm onset, the dayside currents in the postnoon quadrant increase linearly, whereas the nightside currents show a linear increase starting 20-30 min before onset. After onset, the nightside Region 1, Region 2, and nonlocally closed currents and the SuperMAG AL (SML) index follow the Weimer (1994, https://doi.org/10.1029/93JA02721) model with the same time constants in each season. These results contrast earlier contradictory studies that indicate that substorms are either longer in the summer or decay faster in the summer. Our results imply that, on average, substorm FACs do not change with season but that their relative impact on the coupled magnetosphere-ionosphere system does due to the changes in the underlying currents.

AMPERE, Field-aligned currents, Seasonal variability, SOPHIE, Space weather, Substorms
2169-9380
Forsyth, C.
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Shortt, M.
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Coxon, J.C.
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Rae, I.J.
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Freeman, M.P.
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Kalmoni, N.M.E.
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Jackman, C.M.
9bc3456c-b254-48f1-ade0-912c5b8b4529
Anderson, B.J.
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Milan, S.E.
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Burrell, A.G.
121e56d6-1764-4d4a-b02d-cb8ba067aef8
Forsyth, C.
f3d464cf-81c5-4647-b85a-bc3634bdf076
Shortt, M.
78f299ff-c274-4e6a-88a2-bfcfc53317a5
Coxon, J.C.
566baea5-6a30-4855-bde3-a09c115efde4
Rae, I.J.
a9c72922-6a74-4a1c-8bf6-aaa669472c73
Freeman, M.P.
ca34cfc2-086e-4b43-9cb1-921ccbdadb10
Kalmoni, N.M.E.
0ebd24e3-ebef-4498-8ab9-428b956b1437
Jackman, C.M.
9bc3456c-b254-48f1-ade0-912c5b8b4529
Anderson, B.J.
575c5cfc-bf3f-4a16-9cab-4eb584b1835b
Milan, S.E.
4495fdee-b600-43e5-99f7-6193a849b7f5
Burrell, A.G.
121e56d6-1764-4d4a-b02d-cb8ba067aef8

Forsyth, C., Shortt, M., Coxon, J.C., Rae, I.J., Freeman, M.P., Kalmoni, N.M.E., Jackman, C.M., Anderson, B.J., Milan, S.E. and Burrell, A.G. (2018) Seasonal and temporal variations of field-aligned currents and ground magnetic deflections during substorms. Journal of Geophysical Research: Space Physics. (doi:10.1002/2017JA025136).

Record type: Article

Abstract

Field-aligned currents (FACs), also known as Birkeland currents, are the agents by which energy and momentum are transferred to the ionosphere from the magnetosphere and solar wind. This coupling is enhanced at substorm onset through the formation of the substorm current wedge. Using FAC data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet technique, we examine the Northern Hemisphere FACs in all local time sectors with respect to substorm onset and subdivided by season. Our results show that while there is a strong seasonal dependence on the underlying FACs, the increase in FACs following substorm onset only varies by 10% with season, with substorms increasing the hemispheric FACs by 420 kA on average. Over an hour prior to substorm onset, the dayside currents in the postnoon quadrant increase linearly, whereas the nightside currents show a linear increase starting 20-30 min before onset. After onset, the nightside Region 1, Region 2, and nonlocally closed currents and the SuperMAG AL (SML) index follow the Weimer (1994, https://doi.org/10.1029/93JA02721) model with the same time constants in each season. These results contrast earlier contradictory studies that indicate that substorms are either longer in the summer or decay faster in the summer. Our results imply that, on average, substorm FACs do not change with season but that their relative impact on the coupled magnetosphere-ionosphere system does due to the changes in the underlying currents.

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Forsyth_et_al-2017-Journal_of_Geophysical_Research%3A_Space_Physics - Version of Record
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Accepted/In Press date: 9 March 2018
e-pub ahead of print date: 25 March 2018
Keywords: AMPERE, Field-aligned currents, Seasonal variability, SOPHIE, Space weather, Substorms

Identifiers

Local EPrints ID: 420244
URI: https://eprints.soton.ac.uk/id/eprint/420244
ISSN: 2169-9380
PURE UUID: 469e450e-fef0-4c9b-be42-cc4cf191f61c
ORCID for J.C. Coxon: ORCID iD orcid.org/0000-0002-0166-6854

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Date deposited: 03 May 2018 16:30
Last modified: 10 Dec 2019 01:33

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