was reflected back to the surface of the Earth. The height of reflection varies with the electron density since fv is related to electron density, Ne, through the equation: Ne(cm~3) = 1.24 x IO4/? (MHz)2. (1) The heating of the ionosphere with HF waves has been performed for the most part under conditions in which the electron density in the ionosphere was sufficiently great or dense enough to reflect the heater wave. Such ionospheric heating is referred to as overdense heating. In the case of heating resulting from SPS operation, the radio wave giving rise to the heating will pass through the ionosphere. The ionosphere will not be dense enough to reflect the SPS microwave beam. The heating that results in this case is referred to as underdense heating. Because of the frequency involved (2.45 GHz), the heating that the SPS power beam will provide to the ionosphere is believed to be that arising from ohmic losses caused by interactions between the power beam and the electrons, ions, and neutral particles comprised by the ambient ionosphere (6). For sufficiently high frequencies and altitudes, the power density at microwave frequencies can be related to the power density at another frequency by where PSps and/SPS are the SPS microwave power density and frequency and PHf and
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