A3 ionospheric absorption measurements at 1538 kHz — first results

Summary A series of absorption measurements covering more than two years, carried out by the A3 method (oblique incidence) on 1538 kHz at the Panská Ves Observatory, is studied. The measuring equipment is shortly described, as well as calibration and the measuring path. Both the diurnal variation of absorption, including its asymmetry, and the seasonal variation of absorption, including the winter anomaly, are observed in a predicted form. The computed height profile of absorption shows that nearly all the absorption is formed in the middle and upper D-region. The 1538 kHz absorption measurements could serve as a very good tool for studying the D-region, but there are still some difficulties attributed to the determination of the zero level of absorption. Suggestions are given how to remove this difficulty.