The limiting polarization of medium waves reflected from the ionosphere

Abstract

A method is described of measuring the polarization of a downcoming wireless wave which has been reflected obliquely from the ionosphere. Two rotatable crossed loops are adjusted to be in quadrature by dephasing one forwards and the other backwards 45° from resonance. With the usual goniometer technique the search coil gives zero pick-up when the tangent of the angle at which it is set is equal to the ratio of the axes of the projected polarization ellipse, and the frames are turned so that their planes are along the axes of this ellipse. Results obtained on medium-wave broadcasting stations are compared with ellipses calculated on the magnetoionic theory for an assumed angle of emergence and direction of the earth's magnetic field. The comparison shows that, within the present accuracy of the apparatus, the absorption due to electronic collisions does not appreciably affect the limiting polarization of the emergent wave, and sets an upper limit of 106 per sec. to the collisional frequency in the region where the polarization assumes its limiting value. The experiments also suggest that the ratio of the number of ions to the number of electrons in this region is not greater than 10,000, and further work with increased accuracy should give valuable information relevant to the inclusion of the Lorentz term in the magnetoionic theory for the E layer.