ne and thus depends on the ambient D and E layer electron density distributions. The fluid theory results can be used to predict the effects of the SPS beam on the ionosphere and to judge to what extent the Platteville and Arecibo experiments simulate SPS conditions. The predicted changes in electron temperature and density for the SPS peak reference flux of 23mW/cm^ are shown in Fig. 1 and compared with the estimated changes for the Platteville facility operating at 5 MHz X-mode with an effective radiated power of 102MW. It can be seen that Platteville is capable of simulating or exceeding the effects of the SPS beam over a 30 km altitude range, centered near 70 km. A detailed examination of the frequency scaling implied in Eq. (1), the absorption, and the reduction in flux due to spherical spreading shows why Platteville cannot simulate the effects of SPS heating over a larger altitude range. The effective frequency increases below 65 km due to electron-neutral collisions thus reducing the flux below SPS equivalent levels and the combination of nonlinear absorption and spreading loss limit the flux above 95 km. Nevertheless, Platteville will simulate SPS conditions throughout a major portion of the D-layer. In this portion of the lower ionosphere, the electron density will be decreased, rather substantially, after several seconds of heating due to an increase in the electron-temperature- dependent three-body molecular oxygen attachment rate. The electron temperature generally reaches steady-state in less than one second but the density increases build up on a much longer time scale. The changes shown in Fig. lb are after 10 minutes of heating. It is unlikely that sustained heating will take place on a longer time scale since the neutral winds convect the plasma across the beam at speeds of 10-30m/s. The fluid theory estimates of the expected electron temperature increase assume that the energy distribution is Maxwellian. This is questionable since the degree of ionization in the D and E regions is too low for electron-electron collisions to thermalize the population. Following the approach of Engelbart and Phelps , we have also developed a kinetic theory estimate by numerically solving the Boltzmann equation appropriate to ac heating of slightly ionized air. The integro-differential Boltzmann equation is solved parametrically in E/N and u)/N, where N is the total neutral density, in/2tt is the heating wave frequency and E is the rms electric field. If w is much greater than the electron collision and gyro frequencies, then the energy gain per electron and the form of the distribution function depend on the effective electric field E/w and the neutral composition but not the total density The results of the kinetic theory computations are shown in Figs. 2a and 2b. For a flux of 23mW/cm^, the temperature increase is between a factor of two to three times ambient. The relative importance of the various electron cooling mechanisms is shown in Fig. 2b. At 115 km, the standard concentration of atomic oxygen is about 1.5 times that of molecular oxygen and as a result, a significant fraction of the energy is dissipated in 0(^P) fine structure excitation.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==