as high as about 100MHz. The square root term takes the positive sign for the ordinary wave and the negative sign for the extraordinary wave. 6.5.11 Sunspots 6.5.7 The critical frequency H.F. propagation is strongly influenced by sunspot activity. It can cause loss of communication or permit very extensive communica- The maximum electron density of an ionospheric layer determines tion. The presence of sunspots is accompanied by a large increase the critical frequency fckHz. This is the highest frequency for which in ultraviolet radiation. a vertically incident ray can be returned to Earth. This corresponds Sunspots vary cyclicly in number and size with a period of about to a refractive index of zero. The Earth's magnetic field and colli- 11 years. This activity is approximately quantified by the relative sions can usually be neglected, resulting in Equation 6.90. Zurich sunspot number, also called the Wolf number, derived from observations of the number and size of sunspots. (6.90) 6.5.12 Prediction of field strengths, received via the ionosphere 6.5.8 lonospheric cross modulation So far, descriptions have been given of a variety of Ionospheric The oscillatory energy given to an electron by a wave is normally phenomena. This is helpful in qualitative terms, but most engineers small compared with the ambient thermal energy but, for a very require an ability to make quantitative predictions of the field strong wave, this may not be SO. This increases the probability of strength likely to be received in given circumstances. Such calcula- electrons colliding with molecules, SO that all other waves entering tions should allow for each of the ionospheric features, which means the ionosphere will have increased attenuation. If then, the strong they are complicated and lengthy. It is, therefore, not practicable to wave is amplitude modulated, the ionospheric attenuation will vary give a comprehensive description of the calculations here, but with the instantaneous amplitude of this wave, and thus the modu- references lis