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Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 3. Modelling

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 3. Modelling
Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 3. Modelling

This is the third in a series of papers that investigate the semi-annual, annual and Universal Time variations in the magnetosphere. In this paper, we use the Lin et al. (2010) empirical model of magnetopause locations, along with the assumption of pressure equilibrium and the Newtonian approximation of magnetosheath pressure, to show that the equinoctial pattern arises in both the cross-tail current at the tail hinge point and in the total energy stored in the tail. The model allows us to study the effects of both dipole tilt and hemispheric asymmetries. As a test of the necessary assumptions made to enable this analysis, we also study simulations by the BATSRUS global MHD magnetosphere model. These also show that the reconnection voltage in the tail is greatest when the dipole tilt is small but this only applies at low solar wind dynamic pressure pSW and does not, on its own, explain why the equinoctial effect increases in amplitude with increased pSW, as demonstrated by Paper 2. Instead, the effect is consistent with the dipole tilt effect on the energy stored in the tail around the reconnection X line. A key factor is that a smaller/larger fraction of the open polar cap flux threads the tail lobe in the hemisphere that is pointed toward/away from the Sun. The analysis using the empirical model uses approximations and so is not definitive; however, because the magnetopause locations in the two hemispheres were fitted separately in generating the model, it gives a unique insight into the effect of the very different offsets of the magnetic pole from the rotational pole in the two hemispheres. It is therefore significant that our analysis using the empirical model does predict a UT variation that is highly consistent with that found in both transpolar voltage data and in geomagnetic activity.

Lockwood, Mike
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Owens, Mathew J.
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Barnard, Luke A.
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Watt, Clare E.
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Scott, Chris J.
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Coxon, John C.
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McWilliams, Kathryn A.
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Lockwood, Mike
d4b01615-f1c3-4fef-9e54-afaa976c3584
Owens, Mathew J.
e9a2af67-7cbe-4608-8066-1cb2e3327f20
Barnard, Luke A.
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Watt, Clare E.
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Scott, Chris J.
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Coxon, John C.
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McWilliams, Kathryn A.
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Lockwood, Mike, Owens, Mathew J., Barnard, Luke A., Watt, Clare E., Scott, Chris J., Coxon, John C. and McWilliams, Kathryn A. (2020) Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 3. Modelling. Journal of Space Weather and Space Climate, 10, [61]. (doi:10.1051/swsc/2020062).

Record type: Article

Abstract

This is the third in a series of papers that investigate the semi-annual, annual and Universal Time variations in the magnetosphere. In this paper, we use the Lin et al. (2010) empirical model of magnetopause locations, along with the assumption of pressure equilibrium and the Newtonian approximation of magnetosheath pressure, to show that the equinoctial pattern arises in both the cross-tail current at the tail hinge point and in the total energy stored in the tail. The model allows us to study the effects of both dipole tilt and hemispheric asymmetries. As a test of the necessary assumptions made to enable this analysis, we also study simulations by the BATSRUS global MHD magnetosphere model. These also show that the reconnection voltage in the tail is greatest when the dipole tilt is small but this only applies at low solar wind dynamic pressure pSW and does not, on its own, explain why the equinoctial effect increases in amplitude with increased pSW, as demonstrated by Paper 2. Instead, the effect is consistent with the dipole tilt effect on the energy stored in the tail around the reconnection X line. A key factor is that a smaller/larger fraction of the open polar cap flux threads the tail lobe in the hemisphere that is pointed toward/away from the Sun. The analysis using the empirical model uses approximations and so is not definitive; however, because the magnetopause locations in the two hemispheres were fitted separately in generating the model, it gives a unique insight into the effect of the very different offsets of the magnetic pole from the rotational pole in the two hemispheres. It is therefore significant that our analysis using the empirical model does predict a UT variation that is highly consistent with that found in both transpolar voltage data and in geomagnetic activity.

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Accepted/In Press date: 11 October 2020
Published date: 7 December 2020
Additional Information: Funding Information: Acknowledgements. The global MHD simulation results were obtained using BATSRUS, developed by the Center for Space Environment Modeling, at the University of Michigan with funding support from NASA ESS, NASA ESTO-CT, NSF KDI, and DoD MURI. The authors are also grateful to the staff of The International Service of Geomagnetic Indices (ISGI), France and collaborating institutes for the compilation and databasing of the am index which were downloaded from http://isgi.unistra.fr/data_download.php and to the staff of the Space Physics Data Facility (SPDF) at NASA’s Goddard Space Flight Center for the Omni composite of interplanetary observations (made available by SPDF from https://omniweb. gsfc.nasa.gov/ow_min.html). This work is supported by a number of grants. CEW, MJO, CJS and ML at the University of Reading are supported by STFC consolidated grant number ST/M000885/1 and CEW is also supported by STFC grant ST/R000921/1 and NERC grant NE/P017274/1. The work of ML, LAB and MJO at University of Reading is also supported by the SWIGS NERC Directed Highlight Topic Grant number NE/P016928/1/. The work of JCC at the University of Southampton is supported by the UK Natural Environment Research Council (NERC) grant number NE/L007177/1 and by Science and Technology Facilities Council (STFC) Ernest Rutherford grant ST/L002809/1 and Consolidated grant ST/R000719/1. Funding for KAW at University of Saskatchewan was provided by the Canadian Foundation for Innovation (CFI), the Province of Saskatchewan, and a Discovery Grant from the Natural Sciences and Engineering Research Council (NSERC) of Canada. Initial work by KAW for this paper was carried out at University of Reading on sabbatical leave from University of Saskatchewan. The editor thanks two anonymous reviewers for their assistance in evaluating this paper. Publisher Copyright: © M. Lockwood et al., Published by EDP Sciences 2020. Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

Identifiers

Local EPrints ID: 448579
URI: http://eprints.soton.ac.uk/id/eprint/448579
PURE UUID: 0b4c1cd3-11f1-49b2-8014-afae77f2c21f
ORCID for John C. Coxon: ORCID iD orcid.org/0000-0002-0166-6854

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Date deposited: 27 Apr 2021 16:43
Last modified: 26 Nov 2021 03:03

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Contributors

Author: Mike Lockwood
Author: Mathew J. Owens
Author: Luke A. Barnard
Author: Clare E. Watt
Author: Chris J. Scott
Author: John C. Coxon ORCID iD
Author: Kathryn A. McWilliams

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