IONOSPHERIC SCINTILLATION OBSERVATIONS WITH RADIO INTERFEROMETRY

Radio astronomical interferometric observations are affected by atmosp heric refraction, being particularly sensitive to inhomogeneities in t he atmosphere. At frequencies below 2 GHz the influences of the ionosp here are significant in radio astronomy, especially for single dish ob servations and for connected element interferometry. Analytical expres sions for the manifestations of weak ionospheric scintillation in radi o interferometric observations, are derived. We indicate which ionosph eric scintillation parameters can be derived from radio interferometri c measurements. It is shown that the baseline dependence of the observ ed amplitude scintillation index implies a direct determination of the height of the region of random irregular electron distribution. Furth ermore, the linear scale of the irregularities causing scintillation c an be determined directly from the baseline dependence of the scintill ation index S4. From the mean square phase fluctuations as a function of interferometer baseline, the spatial scale of the irregularities re sponsible for this effect can also be determined. From a comparison wi th observational mid-latitude data we find indications that scintillat ion irregularities occur in the lower parts of the F2-layer. The spati al scale of irregularities causing amplitude scintillation is of the o rder of about 25 to about 500 metres. Phase scintillations are caused by irregularities with dimensions which are an order of magnitude larg er.