Wharton, S. J., Wright, D. M., Yeoman, T. K., James, M. K. and Sandhu, Jasmine (2019) The Variation of Resonating Magnetospheric Field Lines With Changing Geomagnetic and Solar Wind Conditions. Journal of Geophysical Research: Space Physics, 124 (7). pp. 5353-5375. ISSN 2169-9380
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Abstract
Standing ultralow frequency waves redistribute energy and momentum around the Earth's magnetosphere. The eigenfrequencies of these standing waves can be measured by applying the cross‐phase technique to ground magnetometer data. To make a detection, the flux tubes in the vicinity of the magnetometers must all be driven at their local eigenfrequencies by a source with a sufficient frequency width. Therefore, successful measurement of the local eigenfrequencies indicates that a broadband source is exciting the flux tubes. We have analyzed 10 years of magnetometer data with an automated cross‐phase algorithm and used correlations with the OMNI data set to understand under what conditions broadband excitation occurs and how the conditions affect the eigenfrequency values. This is the largest such survey of its kind to date. We found that lower eigenfrequencies at higher latitudes (L>5) and higher eigenfrequencies at lower latitudes (L<4) were excited under different conditions. It was also possible to directly compare the first and third harmonics at midlatitudes. The lower eigenfrequencies were excited during more disturbed conditions, and we suggest that these harmonics are driven by solar wind pressure pulses or the Kelvin‐Helmholtz instability at the magnetopause. The higher eigenfrequencies were excited when the magnetosphere was relatively quiet, and we suggest that the cause was waves generated upstream of the Earth's bow shock. The eigenfrequencies were observed to decrease in the middle magnetosphere during disturbed intervals. This is because the intensification of the ring current weakens the magnetic field. Variations in magnetic local time and latitude were also investigated.
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| Additional Information: | Funding information: We would like to acknowledge some very helpful conversations with T. Elsden, I. J. Rae, and C. Forsyth. S. J. W. was supported by NERC Studentship NE/L002493/1. T. K. Y. was supported by STFC Grant ST/H002480/1 and NERC Grant NE/K011766/1. M. K. J. was supported by STFC Grant ST/H002480/1. J. K. S. was supported by STFC Grant ST/N000722/1 and NERC Grant NE/P017185/1. The authors would like to thank the IMAGE magnetometer team for providing the data. The data are available at http://space.fmi.fi/image/beta/ website. The OMNI solar wind data are publicly available from the NASA Space Physics Data Facility, Goddard Space Flight Center (http://omniweb.gsfc.nasa.gov/ow.html). This research used the SPECTRE High Performance Computing Facility at the University of Leicester. |
| Subjects: | F300 Physics F500 Astronomy |
| Department: | Faculties > Engineering and Environment > Mathematics, Physics and Electrical Engineering |
| Depositing User: | Elena Carlaw |
| Date Deposited: | 08 Apr 2021 12:23 |
| Last Modified: | 31 Jul 2021 15:47 |
| URI: | http://nrl.northumbria.ac.uk/id/eprint/45882 |
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