Measurement of the Interstellar Magnetic Field

Abstract

The discovery of linear polarization of the radio emission from the pulsating radio stars¹ provides an opportunity for the first direct measurement of the average magnetic field over distances of up to 100 pc from the Sun. It has already been shown^2,3 that the frequency dispersion in arrival time of the pulses can be entirely explained by a dispersion in group velocity in ionized interstellar gas, and for the pulsating star CP 1919 the integrated electron density ∫N dl along the line of sight has been found³ to be 12·55 cm^-3 pc. If a magnetic field has a component H ₁₁ along the line of sight, then the Faraday rotation of the plane of polarization is a measure of ∫N H ₁₁ dl; the ratio of the two integrals then gives H ₁₁, the average value of H ₁₁ along the line of sight, weighted according to the electron density.