• Microwave beam energy transport is £230 w/m2 at 12.2 cm X • This is less than energy transport through atmosphere due to natural sources Daytime Sunlight £1000 w/m2 0 £10“3 cm X Nighttime Earthlight £ 300 w/m2 @ £10~2 cm X • Absorption of photon radiation in atmosphere is primarily due to particles (dust, water droplets). • This absorption decreases as (particle radius/radiation X) decreases (Mie scattering and absorption). • Therefore microwave heating of atmosphere is negligible (less energy transport, less absorption than due to natural causes). The microwave energy will product only negligible heating in the atmosphere because the 230 cm2 £S less than either the daytime sunlight (<1400 w/m2) or the nighttime earth radiation (£300 w/m2). Moreover, the atmospheric heating is primarily due to particles in the atmosphere (dust, water drops), and the Mie scattering and absorption by such particles decreases as the particle radius/radiation wavelength decreases. Therefore, a smaller fraction o£ the microwave energy at 12.2 cm wavelength will be absorbed than will be absorbed from sunlight (0.2 p to 5 p wavelengths) or earth radiation (5 p to 100 u wavelengths). Finally, the effects of the microwaves on the earth's ionosphere will be much less than that due to other RF sources (Figure 6.3-1). The ionosphere consists of a number of layers (D, E, Fl, and F2) with electron densities £10^ cm"2 lying at altitudes of ^80 km to ^300 km. The electron temperature is £2000°K during the day increasing with altitude, but only ^1000°K at night, independent of altitude. These parameters lead to small DeBye lengths (£10 cm) and fairly low plasma frequencies (£9 MHz) for the daytime F2 layers. There are variations of in these quantities due to factor of three variations in electron density due to earth latitude and solar activity. The disturbances of the earth's ionosphere can be summarized as follows: • Maximum electron density in ionosphere is ^10$ e/cm3 (peak of daytime F2 layer at r^300 km altitude) . • Under these conditions plasma frequency is ^9 x 10^ Hz: Frequencies £ plasma frequency reflected Frequencies £ plasma frequency transmitted
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