sing the phase velocity of the wave, but reducing the 0.8MHz in equatorial regions to 1.6MHz near the magnetic poles. group velocity. Thus, the ionosphere behaves as a medium with a In the UK it is about 1.3MHz. The electrons, under these conditions, refractive index less than one. This value is formulated later when re-radiate with two polarisations, giving rise to the ordinary and other influences have been discussed, but, for now, it is sufficient to extraordinary waves. The ordinary wave propagates as though the know that the refractive index of the ionosphere deviates further Earth's magnetic field were absent, but the electron motion of the from 1 for the lower frequencies, and for the higher electron den- extraordinary wave is in the plane perpendicular to the direction of sities. the Earth's magnetic field. The change of electron density with height results in a refractive index gradient, symbolised by dn/dh. This gradient will cause both 6.5.5 Polarisation coupling loss reflection and refraction of the wave. Reflection dominates when the refractive index changes rapidly over a height range which is The energy in the extraordinary wave suffers more attenuation than small compared with the wavelength. Thus, reflection is the main the ordinary wave in its passage through the ionosphere, in fact it is The sky wave 6/21 largely lost. Also, the ordinary wave may suffer a polarisation 6.5.9 Skip distance and maximum usable change in its passage through the ionosphere and, where the receiv- frequency ing antenna is of the same polarisation as that transmitted, there is a further loss. These factors give rise to the, SO called, polarisation When a wave has a frequency greater than the critical frequency of coupling loss. the layer it can still be returned to earth if the angle of incidence a Polarisation coupling loss is small in temperate latitudes, because is such that inequality 6.91 is satisfied, where no is the refractive the Earth's magnetic field is nearer the vertical. Near the equator, index associated w