A Correlation Method for Measurement of Variable Magnetic Fields

Abstract

A correlation technique is described, which allows a sensible reduction of the noise in power spectrum measurements of the interplanetary magnetic field by means of the search coil magnetometer. Two sensors are required that detect the same signal. If the noises of the two amplifying channels are uncorrelated, the timeaverage of the product of the two channel outputs depends on the power of the magnetic field signal, but is not affected by the noises of the two channels. The application of the proposed technique to space-borne instrumentation leads to a relevant reduction in weight and bulk because of the lower sensitivity required from the sensor. A search coil magnetometer has been developed that is based on the use of sensors of small length (9.5 cm) and that exhibits an equivalent input noise smaller than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> /ƒ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> expressed in γ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Hz as magnetic noise power spectrum.