Although only a small fraction of the beam power is absorbed, it is still significant compared to the natural thermal input into the ionosphere. A rough estimate of the degree of ionospheric modification can be obtained by comparing the electron heat content in the volume of ionospheric plasma where the bulk of the power is absorbed, about 10 km x 100 km, to the energy dissipated in an electron-ion thermal time constant. Thermal equilibrium is reached in about 50 seconds. For a 5 GW system, are absorbed during this interval compared to a heat content of 1 mW-sec. For an incident flux of , the ratio of the ohmic loss to the natural input due to photoelectrons and heat conduction in typical ionospheres ranges from 10 to 40 in daytime and 40 to 160 at night. Thus, one expects significant changes in ionospheric properties as a result. These changes will probably be local and reversible but they nevertheless should be quantified, particularly with regard to sustained operations. Non-local disturbances could occur at high power densities through the generation of acoustic - gravity waves (low frequency sound waves whose behavior is influenced by the earth’s gravitational force). Changes in the neutral composition are unlikely, although the ionic chemistry of the D layer could be altered by high power transmission. It is expected that decimeter radiation with power densities in excess of will cause major changes in the properties of the plasma in the D, E and F regions of the ionosphere. A flux of 100 mW/cm could double the F region electron temperature and cause a 10 to 50 percent local reduction in the electron density. Microwave heating in the D region and the lower E layer will probably reduce the recombination rate and thereby lead to substantial increases in the plasma density (see Appendix C). For example, calculations of D-region heating (see Appendix C) show that a flux of will raise the ambient electron temperature tenfold. This will slow down the dissociative recombination rate by a factor of ten and therefore increase the D-region electron density by as much as 3-4 times the ambient value. (17)
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