HF PRODUCED IONOSPHERIC ELECTRON DENSITY IRREGULARITIES DIAGNOSED BY UHF RADIO STAR SCINTILLATIONS

HF-waves incident on an overdense (HF-frequency < penetration frequency) ionosphere are known to produce large scale electron density irregularities. It is predicted that similar irregularities are formed during underdense HF-modification. The propagation of UHF radio waves originating from radio stars will be affected by such irregularities in the ionosphere. The interest in a scintillation experiment is twofold. One may obtain information on the electron density irregularies and one may learn about the propagation of radio waves through such a perturbed medium. A thin screen (diffractive) theory is derived which allows to draw conclusions on the electron density irregularities from the intensity fluctuations measured on the ground if the phase perturbations are much less than one radian. Since radio stars suitable for scintillation measurements at UHF are very faint an antenna with a large collection area is required. The observations reported in this dissertation were performed with the 300m diameter spherical reflector of the Arecibo Observatory. Successful observations were performed at 430 MHz and at 1400 MHz. Intensity fluctuations at such high frequencies measured with a large antenna suffer severe filtering in the thin phase screen regime. The theory presented in this dissertation includes these filtering effects. Many observations agree with the predictions of that theory. Some observations indicate that refraction effects have to be included to explain the data. HF-induced electron density irregularities were only observed during overdense heating. Several attempts to detect irregularities at an observing frequency of 430 MHz during underdense heating (HF-frequencies 1.1 to 2 times penetration frequency) failed. All the measurements reported in this dissertation are therefore during overdense heating except for the measurement on the 22 September 1980 whichs shows a transition from underdense to overdense HF-modification. Attempts were made to measure electron density irregularities at an observing frequency of 2380 MHz due to overdense heating but the fluctuations were too weak to be discriminated against instrumental uncertainties. The formation time for the irregularities after the HF-power was turned on was (TURN)25 seconds. Their lifetime after the HF-power was turned off was on the order of hours. The electron density perturbations appear to exceed 1% at times. One observation on 11/12 September 1980 reveals that the density perturbation actually corresponds to a depletion.