Cross-Phase Determination of Ultralow Frequency Wave Harmonic Frequencies and Their Associated Plasma Mass Density Distributions

Latitudinally spaced ground‐based magnetometers can be used to estimate the eigenfrequencies of magnetic field lines using the cross‐phase technique. These eigenfrequencies can be used with a magnetic field model and an assumed plasma mass density distribution to determine the plasma mass density in the magnetosphere. Automating this process can be difficult, and so far, it has not been possible to distinguish between the different harmonics. Misidentification of the harmonic mode will lead to incorrect estimations of the plasma mass density. We have developed an algorithm capable of identifying multiple harmonics in cross‐phase spectrograms, using International Monitor for Auroral Geomagnetic Effects magnetometers. Knowledge of multiple harmonics allows the distribution of plasma mass density to be estimated instead of assumed. A statistical study was performed that showed clear common bands of eigenfrequencies, interpreted as different harmonics. These eigenfrequencies were lowest in the early afternoon and at higher latitudes. There was also a greater occurrence of measurements in the dayside. We then modeled the plasma mass density distribution with a power law characterized by the exponent p and compared the model eigenfrequencies to the data. This suggested that the even modes did not form during the interval of this study. Examination of the harmonic spacing and the high occurrence of the third harmonic supported this suggestion. We attribute the absence of the even modes to the driving mechanisms. Finally, we show that an equatorial bulge in plasma mass density was not present in our study.