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Variations of Field Line Eigenfrequencies With Ring Current Intensity
Author(s) -
Sandhu J. K.,
Yeoman T. K.,
Rae I. J.
Publication year - 2018
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2018ja025751
Subject(s) - earth's magnetic field , ring current , magnetosphere , physics , ionospheric dynamo region , geomagnetic storm , magnetic field , computational physics , field line , field (mathematics) , geomagnetic secular variation , intensity (physics) , line (geometry) , geophysics , quantum electrodynamics , atmospheric sciences , mathematics , geometry , quantum mechanics , pure mathematics
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.