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Tailward propagation of magnetic energy density variations with respect to substorm onset times

Tailward propagation of magnetic energy density variations with respect to substorm onset times
Tailward propagation of magnetic energy density variations with respect to substorm onset times
During geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail (X<≈ -9 RE) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail (X<≈ -9 RE). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time.
2169-9380
4741-4754
Coxon, John C.
566baea5-6a30-4855-bde3-a09c115efde4
Freeman, Mervyn P.
dc974e00-a73d-4d41-a2a8-3c5aebd7a5b5
Jackman, Caitriona M.
9bc3456c-b254-48f1-ade0-912c5b8b4529
Forsyth, Colin
176bc4c5-4e98-4419-ad12-631e231b338b
Rae, I. Jonathan
25677763-0501-4b69-859e-fab930260c69
Fear, Robert C.
8755b9ed-c7dc-4cbb-ac9b-56235a0431ab
Coxon, John C.
566baea5-6a30-4855-bde3-a09c115efde4
Freeman, Mervyn P.
dc974e00-a73d-4d41-a2a8-3c5aebd7a5b5
Jackman, Caitriona M.
9bc3456c-b254-48f1-ade0-912c5b8b4529
Forsyth, Colin
176bc4c5-4e98-4419-ad12-631e231b338b
Rae, I. Jonathan
25677763-0501-4b69-859e-fab930260c69
Fear, Robert C.
8755b9ed-c7dc-4cbb-ac9b-56235a0431ab

Coxon, John C., Freeman, Mervyn P., Jackman, Caitriona M., Forsyth, Colin, Rae, I. Jonathan and Fear, Robert C. (2018) Tailward propagation of magnetic energy density variations with respect to substorm onset times. Journal of Geophysical Research: Space Physics, 123 (6), 4741-4754. (doi:10.1029/2017JA025147).

Record type: Article

Abstract

During geomagnetic substorms, around 1015 J of energy is extracted from the solar wind and processed by the Earth's magnetosphere. Prior to the onset of substorm expansion phases, this energy is thought to be largely stored as an increase in the magnetic field in the magnetotail lobes. However, how, when, and where this energy is stored and released within the magnetotail is unclear. Using data from the Cluster spacecraft and substorm onsets from Substorm Onsets and Phases from Indices of the Electrojet (SOPHIE), we examine the variation in the lobe magnetic energy density with respect to substorm onset for 541 isolated onsets. Based on a cross‐correlation analysis and a simple model, we deduce the following: On average, the magnetic energy density increases approximately linearly in the hour preceding onset and decreases at a similar rate after onset. The timing and magnitude of these changes varies with downtail distance, with observations from the mid‐tail (X<≈ -9 RE) showing larger changes in the magnetic energy density that occur ∼20 min after changes in the near‐tail (X<≈ -9 RE). The decrease in energy density in the near‐tail region is observed before the ground onset identified by SOPHIE, implying that the substorm is driven from the magnetotail and propagates into the ionosphere. The implication of these results is that energy in the near‐tail region is released first during the substorm expansion phase, with energy conversion propagating away from the Earth with time.

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Coxon_et_al-2018-Journal_of_Geophysical_Research__Space_Physics (1) - Version of Record
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Submitted date: 20 December 2017
Accepted/In Press date: 15 May 2018
e-pub ahead of print date: 21 May 2018
Published date: June 2018

Identifiers

Local EPrints ID: 422476
URI: https://eprints.soton.ac.uk/id/eprint/422476
ISSN: 2169-9380
PURE UUID: fcd8e30b-95fe-4512-99df-499f28f66cdb
ORCID for John C. Coxon: ORCID iD orcid.org/0000-0002-0166-6854
ORCID for Robert C. Fear: ORCID iD orcid.org/0000-0003-0589-7147

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Date deposited: 24 Jul 2018 16:30
Last modified: 03 Dec 2019 01:35

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Contributors

Author: John C. Coxon ORCID iD
Author: Mervyn P. Freeman
Author: Colin Forsyth
Author: I. Jonathan Rae
Author: Robert C. Fear ORCID iD

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