The spectrum of radio-star scintillations and the nature of irregularities in the ionosphere

Abstract

An observational study of the spectrum of radio-star scintillations has been made with a view to exploring the small-scale structure of the ionosphere. Observations of the intense source in Cygnus were made near transit at an altitude of about 15°. Three instruments were used simultaneously: 1. (1) a sweptfrequency spectroscope to record the dynamic spectrum of scintillations, 2. (2) a swept-frequency interferometer to study positional deviations at different frequencies, and 3. (3) a triangular spaced-aerial system for studying the lateral size and motion of the pattern on the ground. The commonly occurring types of dynamic spectra are described, and their interpretation in terms of ionospheric phenomena is discussed. From the nature of the spectra it is inferred that most of the fluctuations are due to focusing by single lens-like irregularities, rather than to diffraction at a large number of irregularities. The spectra also show that the scintillation patterns are often dispersed across the ground, different frequencies being focused at different points. This is tentatively attributed to ionospheric gradients which act like huge prisms. Such “dispersing regions” may lie at different levels from the “focusing regions.” The degree of fluctuation shows two maxima, one near midnight (winter) and the other near midday (summer). The size, shape, and motion of the pattern on the ground, as well as the dispersion, also vary throughout the year. In many cases the pattern on the ground is highly elongated, a feature which complicates the determination of the direction and speed of motion. For daytime conditions at least, this elongation indicates marked anisotropy in the ionospheric irregularities.