Variations of Field Line Eigenfrequencies With Ring Current Intensity

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. Plain Language Summary The Earth's magnetic field experiences resonant oscillations of individual magnetic field lines at discrete frequencies, known as eigenfrequencies, which play an important role in transporting energy throughout the Earth's space environment. The frequencies of these oscillations can be highly variable, and a key factor in this variability is the strength of the geomagnetic ring current. In this study, we employ realistic models describing the magnetic field configuration and the spatial distribution of the plasma mass density, which also account for variations with ring current intensity. This allows us to estimate how the eigenfrequencies vary for different levels of ring current strength. We explore changes in the magnitude as well as the spatial distribution. A key result is an observed decrease in the magnitude of the eigenfrequency with increased ring c