This report describes an automated method developed to quickly and precisely determine field line eigenfrequencies from ground magnetometer data using the statistical properties of cross-phase and power ratio spectra. This new automated method allows for routine monitoring of field line resonant frequencies, and hence plasma mass density of the inner magnetosphere, with better temporal coverage and over longer intervals than previous studies, which were limited simply due to the time-consuming nature of making selections by hand. To demonstrate the usefulness of the automated procedure, 2 years of data from a pair of magnetometers in the MEASURE array at L = 1.74 were analyzed, and the annual variation in plasmaspheric density was observed. The equatorial mass density reaches a maximum in the Northern Hemisphere winter, and a minimum in summer, with winter values two to three times higher than in summer. This result confirms previous whistler observational studies and theoretical modeling studies.