Remote sensing the magnetosphere using ground-based observations of ULF waves

Ground-based magnetometers record the signature of ultra-low frequency (ULF) wave energy incident from the magnetosphere. A portion of the signal comes from resonant ULF wave structures that form between the northern and southern ionospheres, known as field line resonances (FLRs) in the near-Earth space plasma. It is well known that the resonant frequency depends on the length, the strength, and the distribution of plasma mass along the geomagnetic field from one ionosphere to the other. Given an accurate description of the geomagnetic field in space, the ULF resonant frequencies may be used to remote sense the plasma mass density in the magnetosphere. Identifying the continuum or more directly driven resonant frequencies is the key to the remote sensing process. Techniques involving amplitude, phase, and single and multi-instrument data are discussed. The procedures developed for identifying FLRs at various locations on the Earth's surface have traditionally been accomplished using magnetometers but may also be applied to other ground-based instrumentation such as high-frequency radars, Doppler sounders and meridian scanning photometers. Data analysis methods are reviewed and the development of new analysis methods is encouraged by providing code to compute the Wavelet transform. Remote sensed estimates of plasma mass density are compared with spacecraft data, leading to a discussion of accuracy and further refinements.