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 are 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.
HAARP Induction Magnetometer
The HAARP Induction Magnetometer was provided by Prof. Kanji Hayashi of the Department of Earth and Planetary Physics, University of Tokyo. This instrument measures the geomagnetic field using three orthogonal sensors aligned along the magnetic north (Bx), magnetic east (By), and vertical (Bz) directions. The data acquisition system samples these signals continuously at a 10 Hz rate with 16-bit resolution, producing timeseries data.
Data processing for this instrument begins by converting the binary timeseries to the HAARP standard netCDF format. These netCDF files contain the same timeseries data, but include scale factors and other metadata to provide self-describing datasets. New datafiles are created each day at 00:00 UTC. Each 24-hour netCDF datafile is approximately 5.6 MB in size, containing 864,000 samples of Bx, By, and Bz.
Spectrograms are produced from the timeseries by computing the Power Spectral Density (PSD) of successive 102.4 second (1024 point) segments of data using the Fast Fourier Transform (FFT) technique. Each 24-hour continuous timeseries yields 844 individual PSD plots with 0.01 Hz resolution from 0 to 5.0 Hz. The spectrograms are visualized using a color/intensity map to produce a 2-D image from the data. The DISLIN Scientific Data Plotting library is used to generate the plots.
Audio files are created by converting the raw 10 Hz sampled waveform (Bx channel) to a WAV file at 44.1 kHz, resulting in a playback speedup of 4410 times. This means 24 hours of data plays back in about 20 seconds and signals near 1 Hz (normally inaudible) are played back at 4.4 kHz. The large 1.7 MB WAV file is compressed into a 150 kB mp3 file using the lame encoder. The Adobe Flash plugin is required for playback.