3.5 FARADAY ROTATION EFFECTS 3.5.1 INTRODUCTION The total polarization twist of a linearly polarized wave is related to the total columnar electron content (TEC) of the ionosphere by the following equation: where and the limits of the integral extend from the lower boundary of the ionosphere to the satellite height. The geometry is shown in Figure 3-10. A useful engineering approximation is obtained by introducing a suitable average value for the factor . Titheridge (14) has shown that accurate results can be obtained by evaluating the M factor at 420 km. This approximation holds under a wide range of model ionospheres and satellite locations. Thus the equation can be rewritten in a simplified form that permits direct scaling of with TEC: Variations in , seasonal, diurnal, during a magnetic storm and as a result of solar activity, can all be inferred directly from the associated changes in electron content. 3.5.2 DIURNAL AND SEASONAL CHANGES Comprehensive, synoptic observations of midlatitude electron content have been compiled from Faraday rotation measurements made by Hawkins and Klobuchar (15) at the AFCRL station at Sagamore Hill, Massachusetts, 70° . 82W, 42° . 63N. The data were collected over a 6 year span from 1967 to 1973 by monitoring the received polarization of 136 MHz cw signals from the geostationary
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