IONOSPHERE/MICROWAVE BEAM INTERACTIONS: ARECIBO EXPERIMENTAL RESULTS Lewis M. Duncan, Los Alamos National Scientific Laboratory, Los Alamos, NM William E. Gordon, Rice University, Houston, TX The microwave power-transmission beam of the Satellite Power System (SPS) is predicted to interact with the earth's ionosphere, potentially affecting numerous telecommunications systems. The physics of these interactions and the associated ionospheric effects were investigated in an experimental program using the Arecibo Observatory's high-frequency ionospheric heating facility. The observatory's principal ionospheric diagnostic is an incoherent- backscatter radar, capable of measuring electron number density, electron and ion temperatures, ion composition, and upper atmospheric winds and conductivity. For these experiments, the radar diagnostics were supplemented by onsite ionosondes and photometers and an off-site coherent scatter radar. This radar, operating at 50 MHz with coherent scatter from 3-m irregularities, was located on St. Croix to study E-region short-scale field-aligned striations within the heated ionospheric volume. An investigation of enhanced electron heating of the lower ionosphere was conducted at several frequencies. The radiated power flux was frequency- scaled SPS-equival ent, although wave absorption reduced the power density delivered as a function of altitude within the ionosphere. Electrontemperature increases of up to a factor of two were observed at 75-km altitude, in general agreement with current theoretical models. Studies of wave sei f-focusing and plasma striation processes in the upper ionosphere were conducted for both over- and underdense ionospheric heating. For the underdense heating, no short-scale plasma striations were detected at either E- or F-region heights. Large-scale (kilometer-size) irregularities are commonly observed for overdense ionospheric heating. The measured growth times and scale sizes agree with thermal self-focusing theory. Preliminary results indicate that large-scale irregularities also developed during nighttime HF underdense ionospheric heating. These irregul arities disappeared abruptly near sunrise. Density variations as large as 2% were observed within the irregularities, with a fading period of several minutes. It is not known if the irregularities result totally from HF sei f-foe using, or if they are an HF-triggered natural spread-F condition. Radio scintillations at 430 MHz were easily detected in association with overdense ionospheric-heating striations, but no effect was seen at 1410 MHz. These results indicate that significant ionospheric effects can be generated by SPS-equivalent heating. However, they indicate that the current SPS microwave-beam power density design limit of 23 mW/cm2 may be well below threshold for producing serious telecommunications impacts.
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