The image below is a time-frequency spectrogram, which shows the frequency content of signals recorded by the HAARP Induction Magnetometer. This instrument, provided by the University of Tokyo, measures temporal variations in the geomagnetic field in the ULF (ultra-low frequency) range of 0-5 Hz. The spectrogram images are produced by computing the PSD (power spectral density) of successive 102.4-second segments of timeseries data, and plotting these spectra as color/intensity slices along a 24-hour scale. If the images below is not too interesting, you might want to look at 02 Sep 2004 for an example of narrowband PC1 pulsations, or 27 Jul 2004 for an example of wideband ULF noise during a magnetic substorm. Expanded plots of the last 12 hours, last 6 hours, last 3 hours, and last 1 hour are also available.
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