The study of the polarization of downcoming radio waves has become increasingly important during the last few years. The early theoretical work of Appleton and of Nichols and Schelleng showed that the earth’s magnetic field was likely to influence radio waves profoundly during their passage through the ionized upper regions of the earth’s atmosphere. The upgoing wave entering these regions would be split into two components of opposite senses of polarization, and each component would experience different attenuations and different group velocities during its passage. These views were supported by the experimental work of Appleton and Ratcliffe, who found that downcoming waves of broadcasting frequencies in England were nearly circularly polarized, the sense of polarization being left-handed to an observer looking along the direction of propagation of the wave, which was chosen practically to coincide with the positive direction of the earth’s magnetic field. Appleton and Ratcliffe showed that such a result was to be anticipated, since one of the two downcoming waves (the
extraordinary
component) would be much more strongly absorbed in the ionized regions than the other, the
ordinary
component. The essential correctness of these views was strikingly confirmed by Green in Australia, working under conditions almost identical with those of Appleton and Ratcliffe, but receiving downcoming waves travelling
against
the positive direction of the earth’s field. Green found that the waves received under these conditions were approximately circularly polarized in the
right-handed
sense. Thus far, experimental work on the subject had been confined to broadcasting frequencies, the frequency change device of Appleton and Barnett being employed in the measurement of the polarizations. Theoretical considerations showed, however, that for short radio waves, of frequencies above the critical gyromagnetic frequency of about 10
7
radians per second, it might be possible to receive both the ordinary and the extraordinary components with comparable intensities. These two components were identified experimentally for the F region by Appleton and Builder, who used the pulse or echo method originally due to Breit and Tuve, at a frequency of 3⋅75 mc./sec. Subsequent intensive examination of the ionosphere by the pulse method on short wave-lengths revealed great complexities. At least four regions capable of reflecting radio waves have been discovered, and each of these regions may give rise to multiple echoes, not only for the reasons given above but also because of multiple reflexions between the ground and each region, and between the regions themselves. For these reasons the pulse method of ionospheric exploration has largely superseded the frequency change method in the last few years, since elaborate harmonic analysis is necessary if the latter method be employed when several downcoming waves are present.
The polarization of radio echoes
