Abstract
AbstractWe 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.
Highlights
Shear Alfvén waves are defined as transverse oscillations of magnetic field lines occurring in a plasma
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
It has been demonstrated that FLRs play a key role in the energization of the radiation belts during geomagnetic storms (Baker, Pulkkinen, Li, Kanekal, Ogilvie, et al, 1998; Baker, Pulkkinen, Li, Kanekal, Blake, et al, 1998; Green & Kivelson, 2001; Mathie & Mann, 2000; Nakamura et al, 2002; O’Brien & Moldwin, 2003; Rostoker et al, 1998), and establishing how the spatial distribution of eigenfrequencies varies with geomagnetic activity will provide further insight into the energization interaction
Summary
Shear Alfvén waves are defined as transverse oscillations of magnetic field lines occurring in a plasma. The standing Alfvén waves, commonly termed FLRs (field line resonances), occur at resonant frequencies, or eigenfrequencies, that are controlled by the magnetic field and plasma properties (Dungey, 1954a; 1954b; Southwood, 1974). The magnetic field and plasma properties are highly variable, both in L shell, MLT (magnetic local time), and for different levels of geomagnetic activity. It has been demonstrated that FLRs play a key role in the energization of the radiation belts during geomagnetic storms (Baker, Pulkkinen, Li, Kanekal, Ogilvie, et al, 1998; Baker, Pulkkinen, Li, Kanekal, Blake, et al, 1998; Green & Kivelson, 2001; Mathie & Mann, 2000; Nakamura et al, 2002; O’Brien & Moldwin, 2003; Rostoker et al, 1998), and establishing how the spatial distribution of eigenfrequencies varies with geomagnetic activity will provide further insight into the energization interaction
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