• Thermal Runaway — ionospheric heating at a rate faster than normal cooling resulting in an increased electron temperature at D-region heights (80-100 km). • Plasma Striations — formations of irregularities in electron density along magnetic field lines. Other causes may be: • Effluent from the heavy lift launch vehicles needed to construct the SPS which could result in significant depletion of the ionization density in the ionosphere. Such a depletion would have adverse effects on low and high frequency systems. • Multiple power beams could give rise to gravity waves in the neutral atmosphere causing substantial changes in the ionosphere. 4.2.3.1 Ionosphere Heating All of the SPS ionosphere/microwave interactions currently thought to be important are at least initially driven by ohmic heating. Ohmic heating of the ionospheric plasma and the resultant thermal forces may drive secondary phenomena such as an electron thermal runaway or thermal self-focusing of the microwave beam. These interactions do not necessarily scale linearly with their driving force (ohmic heating) and may occur on microscopic or macroscopic plasma-scale sizes. Microwave radiation propagating through the ionosphere is collisionally damped by free electrons. Although the fraction of wave energy absorbed by the plasma is expected to be small, because this absorbed energy goes directly into the free electrons whose effective heat capacity is very small, the resulting ohmic heating can significantly affect the local ionospheric thermal budget. Ohmic heating of the lower ionosphere by the SPS microwave beam increases the local electron kinetic temperature, which in turn increases the electron-neutral collision frequency but decreases the electron cooling rate. Although collisions may prevent the formation of field-aligned irregularities in the lower ionosphere, plasma striations are a potential source
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