Seasonal and Temporal Variations of Field-Aligned Currents and Ground Magnetic Deflections During Substorms

Forsyth, C., Shortt, M., Coxon, John, Rae, Jonathan, 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, 123 (4). pp. 2696-2713. ISSN 2169-9380

JGR Space Physics - 2018 - Forsyth - Seasonal and Temporal Variations of Field‐Aligned Currents and Ground Magnetic.pdf - Published Version
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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, 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.

Item Type: Article
Additional Information: Funding Information: Natural Environment Research Council (NERC). Grant Numbers: NE/L007495/1, NE/M00886X/1, NE/L006456/1, NE/L007177/1 NERC IRF. Grant Number: NE/N014480/1 Science and Technology Facilities Council (STFC). Grant Numbers: ST/N000722/1, ST/L002809/1, ST/N000749/1 STFC Ernest Rutherford Fellowship NSF. Grant Numbers: ATM-0739864, AGS-1420184
Uncontrolled Keywords: AMPERE, field-aligned currents, seasonal variability, SOPHIE, space weather, substorms
Subjects: F500 Astronomy
F800 Physical and Terrestrial Geographical and Environmental Sciences
Department: Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering
Depositing User: Rachel Branson
Date Deposited: 24 Feb 2022 11:44
Last Modified: 24 Feb 2022 11:45

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