Note on the geometry of the Earth magnetic field useful to Faraday effect experiments

Abstract

The geometric factor (M) involving the earth's magnetic field in the Faraday effect equation is studied. A total of 48 coefficients is used to compute this factor. Some representative curves are shown. It is found that in general this factor is not a constant for a given ray. However, when a ray passes near the saddle point of the ‘M-surface’ M can be assumed to be constant along the ray with reasonable accuracy. In other regions of the sky an empirical average value of M must be used for a given ray and its value depends on the direction of the ray and the a priori ionization density profile. The results are compared with values computed by using dipole earth field.