On the nature of ULF wave power during nightside auroral activations and substorms: 1. Spatial distribution

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[1] We present the results of a statistical analysis of ground-based magnetometer Ultra Low Frequency (ULF) wave power and polarization during substorm expansion phase onset in three ULF wave bands, the longer-period Pi1 (10–40 s) band, the Pi1-2 (24–96 s) band, and Pi2 (40–150 s) wave band, in order to determine whether these wave bands are statistically disparate phenomena during expansion phase onset. Utilizing over 800 nightside auroral activations and substorms identified by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite, we characterize the two-dimensional spatial distribution of ULF wave power, angle of azimuth, and ellipticity with respect to the spatial and temporal onset of the initial auroral brightening as observed by IMAGE. We determine the statistical ULF wave power spectra observed during substorm expansion phase onset and characterize the spatial decay scales of ULF wave power, in each of the three ULF wave bands, as a function of latitude and longitude. In general, we find that the spatial distribution of ULF wave power, angle of azimuth, and ellipticity in each of the three ULF bands is consistent with previous case studies and that the Pi1-2 and Pi2 wave bands are remarkably similar. Additionally, we find that the spatial decay scales of ULF wave power in the long-period Pi1, the Pi1-2, and the Pi2 bands are surprisingly similar. Finally we show that the statistical ULF wave power spectrum is characteristic of a power law with no preferred frequency or discontinuity to differentiate between the three ULF wave bands, demonstrating the importance of studying the entire ULF spectrum during substorm expansion phase onset.