APPENDIX A RADIO WAVE DIFFRACTION BY RANDOM IONOSPHERIC IRREGULARITIES A.1 INTRODUCTION The occurrence of small scale irregularities in the electron density is known to be a normal feature of the ionosphere. One of the main effects of an irregular ionosphere on a radio wave passing through the medium is to impose fluctuations in the wave field. These fluctuations, referred to as scintillations, are present in the amplitude, phase and angle-of-arrival of the signals received from distant sources such as radio stars and satellites. The magnitude of the scintillations, however, are dependent upon the operating frequency, angle of incidence of the wave and the characteristics of the electron density irregularities. The irregularities responsible for the radio wave scintillation are believed to be confined to the F-region ) of the ionosphere and are aligned along the direction of the earth's magnetic field. When the radio wave source is at large angles from zenith, approaching the horizon, it is possible that irregularities in the E-region at the heights near 100 km, become important. Observations of the radio wave diffraction pattern produced on the ground can be used to deduce the characteristics of the ionosphere irregularities or conversely, the wave field can be described provided the source location and the medium properties are specified. The electron density irregularities and likewise the diffraction pattern are both described usually in terms of their statistical properties. The diffraction pattern, for instance, is described in terms of the rms fluctuations in amplitude and phase and their spatial and temporal correlation functions. The problem of describing the fluctuations imposed on a radio wave traversing an irregular medium may be treated theoretically using two different approaches, namely the diffraction method and the scattering method. In the diffraction method, the medium is considered to be equivalent to a certain thin diffracting screen. Because the absorption in the ionosphere is negligible for the frequencies normally used for the scintillation observations, the diffracting screen will produce variations only in phase but not in amplitude across the emerging wavefront. The
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