For different altitudes in the Earth's atmosphere different effects become dominant. Figure 6.1 Atmospheric Attenuation versus Frequency at Sea-Level for Horizontal Propagation Interactions with the Ionosphere Due to the low densities of molecules in the ionosphere molecular absorption losses and scattering effects are negligibly small. Therefore the ionosphere-beam interaction can be divided into two general categories: resistive (ohmic) heating effects and self-focusing instabilities. Ohmic heating is given by the following expression (Duncan and Gordon, 1977): where Q is the volume heat source, Eo is the peak electric field amplitude, f0 is the radio wave operating frequency, fp is the plasma frequency, and vej and ven are the electron-ion and electronneutral momentum transfer collision frequencies, respectively. As one can recognize, the ohmic heating is directly proportional to the inverse square of the radio wave frequency, therefore the ohmic heating decreases from 2.45 GHz to 35 GHz by a factor of about 200. Theories predict that effects like thermal runaway (Perkins et al„ 1977) and self focusing (Duncan ,1977) might occur at power densities of 200 W/m2 and more at 2.45 GHz. (This will be slightly exceeded by a 5 GW power plant in GEO.) On the other side, experiments that have been conducted at the Arecibo radio telescope (Duncan et al., 1977) in order to investigate the effect of a 5 GW microwave beam at 2.45 GHz on the ionosphere showed no changes in electron density and temperature. Experiments have been
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