2.1 Resistive Heating Solar photoionization in the ionosphere produces free electrons with an effective temperature usually exceeding that of the background neutral gas. As electrons gain energy through solar photoionizations, they also lose energy by collisions with the much heavier atoms and molecules of this background gas. The electron temperature is therefore an energy balance between these heating and cooling processes. The collisional heating and cooling interactions of the ionospheric plasma are all dependent on the electron temperature. Under certain conditions the rate of heating may temporarily dominate the normal cooling losses, initiating a rapid increase in the electron temperature that continues until compensating cooling processes develop, limiting the temperature rise. This enhanced electron heating can affect the electron-ion recombination rates, changing ionospheric densities, or drive secondary nonlinear ionospheric interactions, further disturbing the ambient plasma. This section summarizes the threshold conditions required to excite enhanced electron heating in the ionosphere through the collisional damping of electromagnetic radiation. The electron energy balance in the ionosphere is the difference of the energy input from the available sources and the energy losses to the various sinks. For a Maxwellian electron energy distribution, the energy is proportional to the electron temperature, which may be used as an indicator of the energy changes. Clearly, whenever the energy input rate exceeds the cooling rate, the electron temperature must increase. Conversely, in the absence of additional heat sources, the electron temperature will relax to its ambient level. As an electromagnetic wave propagates through a plasma, free electrons respond to the wave’s oscillating electric field. If, while under the action of this wave, the electron suffers a collision, it will scatter out of the electric field, taking with it a part of the wave’s energy. This collisional damping of the electromagnetic wave results in ohmic heating of the plasma. The rate of energy input to the atmospheric free electrons resulting from the absorption of microwave or underdense radio-frequency radiation can be attributed entirely to this ohmic heating process. The corresponding heat source function is given by F2 f2 <2'=^^^ + ^, (1) where E is the wave electric field amplitude, f„ is the local plasma frequency,/is the electromagnetic wave frequency, and vei and ven are the electron-ion and electronneutral collision frequencies. In the lower ionosphere, the electron-neutral collision frequency dominates. In the collision-dominated lower ionosphere, thermal conduction is not an important cooling mechanism. The most effective kinds of energy-transfer collisions are inelastic interactions with O2 and N2, producing rotational and vibrational excitation, and collisions with atomic oxygen, producing excitation of hyperfine levels of the 3P ground state. All of the losses are proportional to the electron density and the number density of the colliding neutral. The temperature dependence provides the interaction effects. The rotational losses and the losses associated with the excitation of the hyperfine structure are also proportional to the temperature difference of the neutrals and electrons and the square root of electron temperatures. The vibrational losses have more complicated dependencies on electron temperature (6).
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