Daytime scintillations induced by high‐power HF waves at Tromsø, Norway

Abstract

During March 1984 the high‐power HF heating facility located at Ramfjordmoen (69.6°N, 19.2°E geographic) near Tromsø, Norway, was used to modify the ionospheric F region in the daytime. The intensity and phase scintillations of 250‐MHz transmissions from the quasi‐stationary polar beacon satellite were measured when the ray path from the observing site to the satellite intercepted the modified ionospheric volume. Narrow band spectral enhancements corresponding to an irregularity scale length of 750 m were detected in the intensity spectra when the radiated HF power developed an estimated power density of about 0.3 mW/m² at the height of reflection. Spectral enhancements at larger scales were not detected in the phase spectra. From the growth and decay of the intensity spectral enhancements during the successive 10‐min “on” and 10‐min “off” periods of the heater the e‐folding growth and decay times of ∼750 m irregularities were estimated to be on the order of 30 s and 2 min, respectively. The threshold power densities required for the generation of the observed irregularity scale sizes were calculated from the self‐focusing instability theory of Cragin et al. (1977) by the use of ionospheric background parameters measured by the EISCAT radar. The theoretical estimates were found to be within a factor of 2 of the HF power densities employed in the experiment. The presence of Fresnel oscillations in the intensity and phase spectra were attributed to a limited irregularity layer thickness less than 50 km. By using the formulations of the thin phase screen theory, it was found that the observed intensity scintillations at 250 MHz correspond to irregularity amplitudes (ΔN/N) of 3% with an outer scale of 1 km.