IONOSPHERIC INTERACTIONS The microwave beam/ionospheric interactions may be divided into two categories: resistive (ohmic) heating effects and self-focusing instabilities. The extent of the interactions depend upon the microwave beam intensity, its frequency, the altitude (D, E, cr F region), and the angle between the beam and the earth's magnetic lines. At some threshold power density level, nonlinear interactions between the power beam and the ionosphere have been predicted to occur. These nonlinear effects could possibly degrade existing HF and VHF communications and VLF navigation systems due to radio frequency interference and multipath degrad- tions. There is also concern regarding the effects of the heated ionsphere on the uplink pilot beam phasing signal. In particular the introduction of phase jitter and/or differential phase delays on the pilot beam will degrade the phase control system. Ionospheric studies to-date indicate that thermal runaway in the E-reg ion (110 Kilometers) and nonlinear thermal self-focusing instabilities in the F-region (200 to 300 kilometers) will limit the maximum power density in the beam to less than 23 mw/cm’ (Ref 8 to 10). However this limit of 23 mW/cm2 (which is believed to be correct to within a factor of two) is a theoretical result for underdense heating and has not been verified by experiments. In the lower ionosphere (D and E regions), thermal conduction can be neglected; the electron heating effects are controlled by a balance between local heating and heat loss (cooling) processes. The energy loss mechanisms for the D and E regions may be summarized as follows: A thermal runaway can occur at these altitudes when the amount of heat deposited into a region exceeds its cooling capacity. The studies indicate the electron temperature will increase from 200°K to 1000°K and the electron density will increase by a factor of three when the SPS beam is heating the ionosphere (Ref 8). The electron density variation as a function of altitude for SPS heating is shown in figure 7. In the D and E regions the microwave heating slows the electron/ion recombination rate and the density increases. The corresponding changes in electron temperatures for a Houston, Texas latitude are shown in figure 8 (Ref 8). In the D and E regions the heated volume is confined to that subtended by the SPS power beam. The heating and cooling constants for the D and E regions are short, on the order of 2 to 100 milliseconds, depending upon the altitude. Thus as the heated particles flow out of the SPS beam, the temperatures return to normal very rapidly. There is little or no coupling of the heated electrons to magnetic field lines as shown in figure 8. In the upper ionosphere (F region), thermal conduction is the main heat loss mechanism. The heating phenomenon of interest in this region is thermally induced self-focusing which is predicted to produce field-alined striations. As the power density of the microwave beam increases there will be a slight focusing and defocusing of the wave due to small variations in the index of refraction of the ionospheric medium. The electric field intensity increases as the incident electromagnetic wave refracts into regions of lesser density. Ohmic heating tends to drive the plasma from the focused regions, further amplifying the self-focusing instability. The plasma thus becomes structured into large-scale field-alined striations. As the incident field intensity increases, these striations become narrower. Temperature increases of 200-500°K will appear along the magnetic field lines intersected by the microwave beam as shown in figure 8. The heated region is not confined to that volume intersected by the power beam since the heated particles will now traverse the magnetic field lines. The temperature contour lines are strongly dependent upon the angle, and hence the degree of coupling, between the magnetic field lines and the power beam. The conclusions from the analyses are that only those rectenna sites at lower latitudes would experience nonlinear therral self-focusing heating as determined by an angle of
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