 wave. The presence of the Earth's magnetic field produces an additional force on the electron. The direction of this force is at right angles to the direction of motion of the electron and to the magnetic field. In general, a charged particle moves in a helix around the lines of a magnetic field. The frequency of rotation depends only on the charge of the particle, its mass, and the strength of the magnetic field. Thus, a resonance exists that is called the gyrofrequency. If the wave frequency equals the gyrofrequency, the velocity of the electrons increases to large values and the wave attenuation is increased. The frequency at which this occurs varies between 0.8 MHz in equatorial regions to 1.6 MHz near the magnetic poles. In the United Kingdom it is about 1.3 MHz. Under these conditions, the electrons reradiate with two polarizations, giving rise to the ordinary and extraordinary waves. The ordinary wave propagates as though the Earth's magnetic field were absent, but the electron motion of the extraordinary wave is in the plane perpendicular to the direction of the Earth's magnetic field. The Sky Wave Polarization Coupling Loss The energy in the extraordinary wave suffers more attenuation than the ordinary wave in its passage through the ionosphere, in fact, it is largely lost. Also, the ordinary wave might suffer a polarization change in its passage through the ionosphere and, where the receiving antenna is of the same polarization as that transmitted, there is a further loss. These factors give rise to so-called polarization coupling loss. Polarization coupling loss is small in temperate latitudes because the Earth's magnetic field is nearer the vertical. Near the equator, the magnetic field is nearly horizontal. This means that on north-south paths, there might be as much as 6 dB of coupling loss. The effect is at its greatest on east-west paths near the equator, where the polarization coupling loss can be very large. The Refractive Index of the lonosphere The refractive index of an ionized medium withou