Induction Magnetometer

The induction magnetometer was removed by the Air Force during the site shutdown in June 2014.

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.

Typical signals

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.