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Variations of Field Line Eigenfrequencies With Ring Current Intensity

Sandhu, JK; Yeoman, TK; Rae, IJ; (2018) Variations of Field Line Eigenfrequencies With Ring Current Intensity. Journal of Geophysical Research: Space Physics , 123 (11) pp. 9325-9339. 10.1029/2018JA025751. Green open access

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Abstract

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.

Type: Article
Title: Variations of Field Line Eigenfrequencies With Ring Current Intensity
Open access status: An open access version is available from UCL Discovery
DOI: 10.1029/2018JA025751
Publisher version: https://doi.org/10.1029/2018JA025751
Language: English
Additional information: © 2018 The Authors. This is an open access article under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/3.0/).
Keywords: field line resonances, ULF, ring current, storms, eigenfrequencies, radiation belts
UCL classification: UCL
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Space and Climate Physics
URI: https://discovery.ucl.ac.uk/id/eprint/10063885
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