SPS HEATING IN THE LOWER IONOSPHERE, AN EXPERIMENTAL VIEW Frank T. Djuth, David S. Coco, Daniel A. Fleisch and Suman Ganguly Dept, of Space Physics and Astronomy, Rice University, Houston, Texas 77001 It is predicted that heating by the SPS microwave beam will substantially increase ambient electron temperatures and modify electron density distributions within the ionosphere. In the present study, the incoherent scatter radar at Arecibo Observatory is used to investigate enhanced electron heating in the D and E regions of the ionosphere and irregularity formation at E-region alt i tudes. Initial tests for enhanced electron heating in the E region (95-115 km) were carried out in 1978 using the 430 MHz radar system at Arecibo. This system delivered heating pulses to the ionosphere having power densities of 1.5 mW/cm^ at the center of the radar beam (SPS frequency scaled 50 mW/cm^) at 100 km altitude. The lengths of the heating pulses ranged from 0.4 to 9-0 msec. The 430 MHz radar also served as the principal ionospheric diagnostic in the experiment. Radar signal power, backscattered from a short diagnostic pulse by free electrons in the ionosphere, was recorded as a function of altitude. By comparing power profiles before and after a radar heating pulse was transmitted changes in electron scattering cross section could be deduced. Because of the known temperature dependence of this cross section, the effective electron heating averaged across the beam could be determined. Typically, 100°K increases in electron temperature were observed yielding Te/Tj values of 'v 1.5 at 95 km altitude. Following accurate treatments of the radar power distributions across the heating and diagnostic beams and subsequent improvements in the theoretical calculations, general agreement now exists between theory and these observations. More recently, a series of observations designed to determine the amount of heating that occurs at D-region heights (60-95 km) have been performed. The observations utilized the 430 MHz radar as both a heater and a diagnostic in a manner similar to that described for E-region observations but with some notable changes. The experimental design necessitated that the length of the heating pulse be shortenea to .2 msec. This was compensated for by the shorter rise times anticipated for electron heating in the D region. In addition, electron temperatures were deduced from measurements of the incoherent scatter frequency spectrum rather than total scattered power. These observations yielded values of Te/Tj for D-region heating that peaked near Te/Tj = 2.5 ± .5 at an altitude of 72 km. Finally, an additional series of D-region measurements were carried out at Arecibo using a new auxilliary HF (3~12 MHz) facility to heat the ionosphere. This facility was located 17 km away from Arecibo Observatory and was only partially completed at the time of the measurements. During the observations, the HF facility produced SPS frequency-scaled power densities of 5 mW/cm^ near the center of a 5.1 MHz beam at an altitude of 75 km. The estimated power density takes into account absorption by ionization below 75 km but assumes a 100% radiating efficiency for the HF transmitter. Observations of D-region heating were conducted using an HF frequency of 5.1 MHz and X-mode polarization. In order to measure changes due to electron heating, the HF transmitter was repeatedly cycled on for 1 minute and off for 1 minute. Under these test conditions, maximum heating was observed near 75 km altitude, where Te/Tj was found to be 2.1 ± .5. In addition to the electron temperature measurements, the possibility that heating by an HF wave penetrating the E region might generate ionospheric irregularities was also examined experimentally. Unfortunately, at the time of these observations, the HF facility could be operated only in a pulsed mode
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