Standing Alfvén waves on geomagnetic field lines result in large scale resonant oscillations at frequencies determined by the magnetic field configuration and the plasma mass density distribution along the field. These oscillations are a fundamental mechanism for the transfer of energy and momentum within the magnetosphere.

By applying the cross phase technique to ground magnetometer observations, the eigenfrequencies of field lines can be directly measured. Magnetic field observations from the IMAGE and CARISMA ground magnetometer arrays are analysed, providing simultaneous coverage over a large range of L shells. The direct observations of eigenfrequencies are compared to estimates of eigenfrequencies that are determined using magnetic field and mass density models with a time-of-flight technique.

For quiet geomagnetic conditions, we find that existing magnetic field and mass density models well describe the spatial variations in the observed eigenfrequencies. During storm times, magnetic field and mass density models suggest that the eigenfrequencies are depressed due to an enhanced ring current acting to weaken the inner magnetospheric field. The results are supported by direct observations of the eigenfrequencies, using both large scale statistical studies as well as case studies of geomagnetic storms. This global depression of field line eigenfrequencies has significant implications for the propagation of wave power throughout the magnetospheric system.

In these studies, we have demonstrated the capability of eigenfrequencies to provide a useful diagnostic for magnetic field and plasma conditions within the inner magnetosphere. In particular, during highly dynamic storm times, the eigenfrequencies unveil large scale magnetospheric changes.