The loss of electron energy by thermal conduction out of the beam is slow below 150 km because of the small mean free path. The incident RF beam intersection determines the form of the hot region. Above about 200 km, heat conduction is magnetically confined transverse to the magnetic field direction but is good parallel to the field. Some illustrations from PR are reproduced here, by permission. Figure 1 shows energy loss rates versus height for certain of the atmospheric products, using a daytime model atmosphere with and without irradiation by the assumed RF beam. Large effects near 100 km are to be noted. The assumed satellite beam of 10 GW is taken parallel to the field. Figure 2 shows the daytime intensity distribution of 0( D) 6300A emission near Boulder as affected by the satellite beam. The maximum heating curve takes the beam parallel to the field; the others are for indicated intersections of the beam with the field line at heights of 200, 250, and 350 km. Even with the strong daytime photoproduction, one sees pronounced enhancements caused by the added beam. GREAT LOCAL HEATING So-called runaway thermal heating is an interesting possibility in ionospheric radio-frequency heating, as discussed by Holway and Meltz (1973) and others. The inception of thermal runaway can be noted in ground-based studies with very large antennas. The higher the electron temperature becomes, the greater is the ability to transfer RF energy via collision frequency. The loss rates for electron energy are lessened as electron
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