nd galactic cosmic rays, cause the reflections take place off the lower edge of the E layer. This is rarely gases of the upper atmosphere to become ionised. The degree of used as a means of communication, but is a source of interference to m.f. broadcast services. ionisation does not change uniformly with height, but there are relatively dense regions in layers above the Earth's surface. The At still lower frequencies, reflection off the lower edge of the properties of the four principal layers are: ionosphere occurs before much attenuation takes place. D: About 50km - 90km in altitude. Greatly dependent on sun's 6.5.4 Gyrofrequency - the ordinary and intensity. Absorbs low frequency radio waves on passage extraordinary wave through layer. E: About 90km - 130km in altitude. Depends on sun's intensity - In the absence of a magnetic field, electrons would oscillate in the small residual level at night. Exhibits a 'sporadic' nature. direction of the electric vector of the wave. The presence of the F1: About 130km - 210km in altitude - merges with F2 layer at Earth's magnetic field produces an additional force on the electron. sunset. Depends on sun's intensity The direction of this force is at right angles to the direction of F2: About 210-400km in altitude-merg with F1 layer at sunset. motion of the electron, and to the magnetic field. In general a Influenced by Earth's magnetic field. Depends on auroral charged particle moves in a helix around the lines of magnetic field. zones, particularly in sunspot activity. The frequency of rotation only depends on the charge of the particle, its mass, and the strength of the magnetic field. Thus a resonance 6.5.2 Reflection and refraction of waves in the exists which is called the gyrofrequency. ionosphere If the wave frequency equals the gyrofrequency, the velocity of the electrons increases to large values, and the wave attenuation is Radio waves cause free electrons and ions to oscillate and re-radi- increased. The frequency at which this occurs varies between ate, increa