MICROWAVE HEATING OF THE LOWER IONOSPHERE G. Meltz and W. L. Nighan United Technologies Research Center, East Hartford, Connecticut 06108 The SPS microwave power beam is sufficiently intense to cause large changes in the properties of the lower ionosphere by ohmic heating of the plasma. Although the fraction of power that is absorbed from the beam is very small, it is comparable to the solar heating rate of the neutral gas. Power is absorbed from the beam at a rate that is proportional to the product of the electron density ne and the electron-neutral collision rate v, and the ratio of the intensity S to the square of the microwave frequency f. The peak absorption occurs at an altitude where ne is a maximum. During the day, this is between 75 to 105 km, depending on the time-of-day, season, and solar activity. The maximum ohmic loss rate Qmax is approximately Qmax " 4 x 10 ' (vne/3.9 x 10y s cm J) x (S/23 mW cm“2)(f/2450 MHz) ergs/s - cm”^ using representative values for v and ne at 1^0 km. For comparison, the neutral heating rate is about 9.8 X 10“^ ergs/s-cm . Since the microwave absorption is almost one-half of the solar heating, major changes can be expected in the properties of the D and E-layers of the ionosphere. This paper addresses the development of a predictive model of the underdense interaction of an electromagnetic beam and the lower ionosphere. The interaction is considered to be underdense if the electromagnetic frequency exceeds the maximum plasma frequency throughout the ionospheric region of interest. A self-consistent fluid theory formulation of underdense heating, incorporating the latest information on electron cooling and electron-temperature-dependent reaction rates, has been used to estimate the expected changes in the lower ionosphere due to the SPS beam. A computer code has been developed to integrate the coupled equations for power density, electron temperature, and electron density as a function of altitude and time for both time-varying and steady heating fluxes. The principal electron cooling mechanisms are: (1) rotational excitation of N2 and O2> (2) vibrational excitation of N2 and O2, and (3) excitation of $P fine structure ground state levels of 0. At the base of the D- region, namely at altitudes of 50-75 km, the density will decrease^ due to an increase in the electron-temperature-dependent attachment rate to molecular oxygen. Above 85 km, the density will increase as a result of sustained heating due to a reduction in the recombination rate of 0^ and N0+. The absorption coefficient and the corresponding ohmic loss are both inversely proportional to the square of an effective frequency fe defined by fe = f[(l - fB/f cos 0)2 + (v/2irf)2]X^2 (1) in terms of the gyrofrequency fg, 0 the angle between the propagation direction and the magnetic field , and v an effective Appleton-Hartree collision frequency. It can be seen from (1) that unless (fg/f) 'cos0<<1 and (v/2irf)<<l, ohmic loss and absorption will not scale simply as l/f^. In general, the scaling from SPS to HF frequencies is quite complicated and nonlinear since an increase in temperature changes v and thus fe. The scaling is further complicated by the fact that the absorption coefficient is directly proportional to
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