HAARP Induction Magnetometer
Spectrogram Processing
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.
All data processing is performed on a GNU/Linux system in the HAARP Diagnostic
Data Center using C++ and
Python routines.
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