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Page updated December 10, 2007
HAARP Induction Magnetometer
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 overall 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.
Magnetic field variations of interest in this program are those induced by
electric currents in the ionosphere. The major signal categories detected by
the induction magnetometer are short period magnetic pulsations such as Pc1,
Pc2, Pc3, PiB, and PiC in a frequency range above a few tens of milliHertz.
Among these, the induction magnetometer most efficiently detects Pc1 waves
in the frequency range from 0.1 Hz to 3 Hz. Pc1 signals are the result of
ion-cyclotron radiation generated near the equatorial plane of the
outer-magnetosphere that make their way to the ionosphere guided by the magnetic
lines of force. In addition, signals generated in the atmosphere that are caused by
lightning discharges, the Schuman resonances, are also detected and sometimes
become strong enough to mask signals from the ionosphere.