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About the Instrument
The induction magnetometer detects temporal variation of the geomagnetic field based on Faraday's law of magnetic induction. This instrument, which was provided by the University of Tokyo, is composed of three individual sensors. Each sensor is comprised of a large number of turns of fine copper wire wound around a rod with high magnetic permeability. (See a photograph of the sensor and its construction.) The sensitivity of each sensor is determined by the effective area of the detection coil, that is, the cross sectional area of each winding, and the number of turns, and by magnetic flux density threading the coil. The magnetic flux density is enhanced by a factor of approximately 1,000 by the high-permeability metal core.
The induction magnetometer installed at the HAARP site is designed to detect a signal level of a few picoTesla (pT) at 1 Hz. The over all frequency response of the magnetometer is shaped by Faraday's law at frequencies below 1 Hz and by active filters at frequencies above 1 Hz. Below 1 Hz the coil response is proportional to the time derivative of the magnetic field and thereby gives a response proportional to the frequency. Above 1 Hz, signals are suppressed by a low-pass filter with a corner frequency at 2.5 Hz. The filter response diminishes by 24 dB per octave above the corner frequency and thereby eliminates interference from 60 Hz radiation. The magnetometer sensors are aligned along the magnetic north, magnetic east and vertical directions to form an orthogonal measure of the derivative of the field. The sensor outputs are amplified by 40,000 and sampled at a 10 Hz rate with 16-bit resolution in a full scale of 10 Volts.