Abstract
Mean, integrated geomagnetic noise power spectra in six octaves between 0.002 and 0.128 Hz from the Air Force Geophysics Laboratory (AFGL) magnetometer network were grouped into seasonal ensembles. We found that the local time variation of the mean logarithm of this ensemble, with respect to both 3‐hr and 8.5‐min time blocks, consistently experienced a 5–10 dB maximum centered around local noon at higher frequencies and near dawn and dusk at lower frequencies, a factor of 3–10 in power. The shape of this trend differed with geomagnetic field component, geomagnetic latitude, and frequency octave. The high‐frequency Pc 3 and 4 waves have a broad maximum in power centered on local noon, but the longer‐period Pc 4 and 5 wave power maximizes away from noon toward dawn for the north‐south geomagnetic field component and toward dusk for the east‐west component. Two years of data did not provide a large enough sample to resolve a significant seasonal variation in the shape of the diurnal trend. These trends changed consistently during periods of enhanced global activity, as measured by the index Ap. These are among the first systematic observations of Pc 5 wave power at low geomagnetic latitudes (40°–55°), indicating that wave energy from the outer magnetosphere is coupling or propagating to low‐latitude locations. This statistical study extends the few previous event studies by providing an explicit parametrization of local time trends in wave power as a function of frequency subband, geographic location, and magnetic activity.
Published Version
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