SIMULATED D- AND E-REGION EFFECTS OF THE SPS POWER BEAM Robert L. Showen SRI International, Menlo Park, CA 94025 Since the beginnings of commercial broadcasting, the lower ionosphere has been inadvertently heated. The classical Luxembourg effect, in which the audio modulation from a powerful radio transmitter is weakly transferred to another radiowave passing overhead, is due to electron heating. In this paper the mechanism of the Luxembourg effect will be reviewed, the transmitters used to simulate the solar power satellite (SPS) microwave power beam are identified, and results of various experiments are discussed. The cross modulations observed in the Luxembourg effect are possible because the electrons in the lower ionosphere have a very rapid thermal relaxation time, and suffer a collision frequency proportional to temperature. Since the ohmic absorption of radiowaves is proportional to collision frequency (when the collision frequency is less than the wave frequency), the passing wave will be more absorbed when the electrons are heated. Thus the oscillation of the electron temperature at an audio rate can produce cross-modulation of a wave passing through the heater volume. Cross-modulations measured on certain telecommunications frequencies will be discussed later in this paper. The Platteville, Colorado, HF heating facility (5 to 10 MHz, 2MW into a 20 dB gain vertical antenna) and the old Arecibo, Puerto Rico HF facility (5 to 12 MHz, 100 KW into a 35 dB antenna) have been used to heat the D-, E- and F-regions of the ionosphere. A new Arecibo facility (3-12 MHz, 800 KW into a 23 dB antenna) is currently becoming operational. Additionally, incoherent backscatter transmitters at Arecibo at 40 and 430 MHz have heated the lower ionosphere. All of these transmitters are powerful enough to be equivalent in energy deposition in the D-region to the microwave power beam from the proposed SPS. Results from the Arecibo 40 MHz heating experiment indicate that a simple theory of heating and cooling using the fractional energy loss parameter G can account for the observations. The 40 MHz transmitter with 1.5 MW power gives a power density of .06 W/M2 at 100 km, which is equivalent, after the frequency squared correction, to 225 W/M2 at the SPS frequency of 2.4 GHz. This heating led to a measured electron temperature increase of 100 K - a 50% increase of the ambient 200 K temperature. The thermal relaxation time was also measured to be about 4 msec at 100 km. At lower altitudes the heating becomes greater and the relaxation time shorter due to the increased collision frequency, while, simultaneously, the number of electrons drops so that fewer and fewer electrons are heated more and more. This experiment has been repeated and extended using the 430 MHz transmitter and similar results were obtained. In one set of measurements at the Platteville heater, cross-modulation was detected on four frequencies over a path 50 km to each side of the heated volume, from Ft. Collins to Bennett, CO. The four frequencies were 60 kHz (WWVB), 1410 kHz (KCOL), and 2.5 and 5.0 MHz (WWV). The cross modulation measured was typically .01 to 0.1%, which implies a significant change in electron temperature. Such magnitudes of cross-modulation could not be noticed by users of these broadcasts. For the 60 kHz wave the reflection altitude was only somewhat above 80 km, so this simulation was approximately SPS equivalent.
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