temperature rises and as heat conduction becomes restricted at low heights, and it is thereby quite possible to produce a localized great enhancement of temperature. Figure 3 from the PR calculations for Arecibo illustrates that the temperature at low altitudes rises faster with beam power than it does at higher altitudes. With sufficient and attainable power, the assumed loss processes are unable to control the increasing heat gain. Unbounded runaway temperatures, however, refer only to the modelled processes; actual limiting would occur at sufficiently high temperatures as ignored loss channels become more important. The production of thermal runaway temperatures in the low (50 to 100 km) portion of a strong RF beam is an intriguing, though not catastrophic, result of intense atmospheric RF heating. It is seen in the example of Figure 4, which gives the calculated meridional distribution of T at Boulder for various intersection heights of the beam with the field line. Temperature rises of 700°K are indicated at the high F region electron concentrations, though good conduction is a mediator. The runaway effect, however, is shown below 100 km by the unlimited breakthrough of temperature contours. The nonlinear effects that develop in such a regime make accurate calculations unsure. It should be emphasized that no tendency toward runaway becomes noticeable until the beam power is very intense. LOCAL OPTICAL RESULTS The natural excitation of upper atmospheric emissions comes from: • Direct excitation by daytime photons (effectively a few to 20 eV) • Direct photoionization and excitation (10 to 20 eV) • Collisions with secondary photoelectrons (effectively a few to 20 eV) • Dissociative recombination of molecular ions and electrons (several eV) • Chemical reactions yielding excited products (a few eV) • Collisions with energetic electrons (auroral 0.5 to 10 keV, and secondaries) • Collisions with superthermal electrons from plasmasphere (to 100 eV) • Minor contribution from energetic incident ions (0.5 to 30 keV)
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