ULF Wave Eigenfrequencies in the Earth's Magnetosphere

This thesis studies standing ultralow frequency (ULF) waves in the Earth's magnetosphere. In particular, it looks at how to automate the measurement of the resonant frequencies of geomagnetic field lines and how we can use this information to understand the driving processes. The first of the three studies presented in this thesis develops the automated cross-phase algorithm that measures the eigenfrequencies. It was applied to a month of magnetometer data to understand the diurnal and latitudinal variations in eigenfrequency. The algorithm is capable of detecting the higher harmonics of the field line and by solving the magnetohydrodynamic wave equation, it was possible to determine that only the odd harmonics were present. Knowing several harmonic frequencies enabled the estimation of the quantity and distribution of plasma mass density along the field line. The next study modifies the technique for application to SuperDARN radar data. Several techniques were employed to deal with the inconsistent sampling rate. Artificial backscatter was used for testing to remove the uncertainty in the backscatter origin. These techniques made it possible to measure eigenfrequencies with SuperDARN and the results are consistent with those from ground magnetometers. The energy source of the resonant signature was also investigated and found to be broadband in nature. The last study uses the algorithm to understand which sources drive the field line excitation. A 10-year survey of magnetometer data was performed and the results correlated with solar wind data. It was found that the Kelvin-Helmholtz instability and dynamic pressure pulses were responsible for lower frequencies and occur during more disturbed conditions, whereas the ion-cyclotron instability excites higher frequencies during quieter intervals. An enhanced ring current was also found to decrease the eigenfrequencies because of the lower magnetic field strength.