The electron temperature is a sensitive balance between heating processes and cooling interactions. As the electron temperature increases the electron-neutral collision frequency rises, thereby further increasing the ohmic heating. This selfamplifying process must be balanced by electron cooling losses, which also become more efficient as the electron temperature increases above its ambient value. For close-to-ambient electron temperatures in the lower ionosphere, the principal cooling mechanism is rotational excitation of N2 and O2. Above about 100 km, excitation of hyperfine levels in atomic oxygen becomes important, depending on the concentration profile. As the electron temperature increases to several times ambient, vibrational excitation of N2 and O2 provides substantial cooling losses and ultimately saturates the heating. The time it takes a plasma to self-consistently reach an equilibrium between these competing processes is called the heating time scale. After the ionospheric wind has swept the plasma out of the heating beam, the electron temperature relaxes to its ambient level on a cooling time scale not very different from its normal heating counterpart. In the lower ionosphere, both heating and cooling equilibria are reached in a few milliseconds or less. For normal ionospheric heating, the balance between the heat input and cooling loss is continuously converging on some stable equilibrium. For this “stable” heating, the rate of change of electron temperature is decreasing at all times. However, for a sufficiently large heat source function, this criterion is not always satisfied. When the rate of electron temperature change is increasing as a function of time, a condition of “thermal runaway” temporarily exists. When this occurs, the electron temperature and the heating time constant become nonlinearly dependent on the incident electromagnetic wave intensity. A description of this enhanced electronheating process and predictions for ionospheric heating by the solar power satellite microwave power beam are presented in the following section. 2.1.1 Enhanced Electron Heating. Holway and Meltz (6), investigating the effects of strong radio-wave heating of free electrons in the lower ionosphere, first introduced the concept of an electron temperature runaway. Subsequent studies (7), including comprehensive models for the dominant collisional cooling processes, have been performed to predict the enhanced electron-heating thresholds, time dependences, and saturation limits, as functions of the initial ionospheric conditions and the heating wave frequency and power density. These theoretical results can be compared to experimental radio-wave heating observations of the ionosphere, and used to predict the ionospheric effects of proposed systems such as the solar power satellite microwave-power transmission beam. Enhanced electron heating is predominantly a lower ionosphere effect. For SPS microwave beam parameters, the predicted electron temperatures are shown in Fig. 1 as a function of altitude and time. The electron temperature apparently increases within the beam by a factor of 3 to 4. Predicted changes in electron temperature throughout the ionosphere are shown in Fig. 2. In addition to direct temperature effects, the ohmic heating causes a decrease in the rate of electron recombination, resulting in a general slight enhancement in the electron number density. Predictions of the electron density modification due to microwave heating are presented in Fig. 3. No significant telecommunications impacts are expected to accompany these relatively small changes in ionospheric density. The threshold power flux density for exciting enhanced electron heating is altitude dependent, but can be approximated at 20-30 mW/cm2. However, even for smaller
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