In an ionised medium in the presence of a magnetic field, a radio wave is split into two circularly polarised components, each propagating independently. In the ionosphere a radio wave is split by the earth's magnetic field into ordinary (o) and extra-ordinary (x) waves. The partitioning of the wave energy between the two depends on the angle the wave makes with the magnetic field. At low frequencies, the x-wave is heavily attenuated relative to the o-wave.
An Extended Glossary of Solar Terrestrial Terms from IPS Radio & Space Services