Sandhu_et_al-2018-Journal_of_Geophysical_Research%3A_Space_Physics.pdf (2.22 MB)
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Variations of Field Line Eigenfrequencies With Ring Current Intensity

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journal contribution
posted on 12.08.2019, 15:03 by J. K. Sandhu, T. K. Yeoman, I. J. Rae
We present results from the closed magnetosphere (5.9≤L < 9.5 over all magnetic local times) to demonstrate and assess the variations in field line eigenfrequency with geomagnetic activity. Using the time-of-flight technique with realistic magnetic field and mass density models, the spatial distributions of field line eigenfrequencies are determined for a range of different geomagnetic activity levels, as defined by the Dst index. The results indicate that during geomagnetically active conditions, the eigenfrequency of a given field line is generally decreased compared to quiet times, in addition to variations in local asymmetries. By comparing the dependence to changes in the magnetic field and mass density distribution, it is established that the inflation and weakening of the geomagnetic field outweighs decreased plasma mass density and is the sole contributor to decreased eigenfrequencies with increased geomagnetic activity. We highlight the importance of considering the magnetic field, mass density, and average ion mass contributions when using observed eigenfrequencies to probe magnetospheric conditions. Furthermore, the estimates significantly improve upon existing time-of-flight results, through a consideration of mass density changes with geomagnetic activity. We also provide estimates of eigenfrequencies for a comparatively extended spatial region than available from prior direct observations of field line resonances. The results have clear implications for furthering our understanding of how wave energy propagates throughout the magnetosphere during geomagnetic storms.

Funding

. K. S. is supported by STFC consolidated grant ST/N0007722/1 and NERC grant NE/L007495/1. T. K. Y. is supported by STFC grant ST/H002480/1 and NERC grant NE/K011766/1. I. J. R. is supported in part by STFC consolidated grant ST/N0007722/1 and NERC grant NE/L007495/1. This study utilized data from the CARISMA ground magnetometer array, which are available from http://carisma.ca. The authors thank I. R. Mann, D. K. Milling, and the rest of the CARISMA team for data. CARISMA is operated by the University of Alberta, funded by the Canadian Space Agency.

History

Citation

Journal of Geophysical Research: Space Physics, 2018, 123(11), pp. 9325-9339

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Journal of Geophysical Research: Space Physics

Publisher

American Geophysical Union (AGU), Wiley

issn

2169-9380

eissn

2169-9402

Acceptance date

31/10/2018

Copyright date

2018

Available date

12/08/2019

Publisher version

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JA025751

Notes

This study utilized data from the CARISMA ground magnetometer array, which are available from http://carisma.ca.

Language

en