The magnetosphere is a dynamic system full of ultralow frequency Alfven waves. These are perturbations of the magnetic field that guide energy along field lines, much like waves on a string. Alfven waves can be reflected by the ionosphere and hence standing waves are formed with a set of harmonic frequencies. These harmonic frequencies depend on the plasma mass density and magnetic field strength along the field line.
These harmonic frequencies can be measured by applying the cross-phase technique to ground-based magnetometers. This relies on the local magnetic field being driven by a broadband source of energy which excites the local field lines at their respective harmonic frequencies. Hence, measurement of these harmonic frequencies implies the presence of a broadband energy source.
Using ten years of magnetometer data, we have studied under what geomagnetic conditions the field lines will be excited. We find that the first and third harmonics are detected under different conditions and we are able to link each harmonic to a possible source of ultralow frequency wave energy. We also look at how the values of the harmonic frequencies change with these conditions. This can tell us about how the magnetospheric structure is changing.