Radio-star visibility fades observed in the auroral zone

Abstract

The operation of a phase-switch interferometer is analyzed to show that reductions in output may be interpreted as reductions in correlation between signals arriving at the two antennas. Such reductions, therefore, constitute decreases in the visibility of the source under observation. Visibility fades of the radio stars Cygnus A and Cassiopeia A, observed on 223 megacycles in the auroral zone during one year of maximum sunspot activity, are analyzed empirically. Fade-occurrence distributions in time and space are presented and comparisons with certain other geophysical phenomena are discussed. Fades observed on 456 megacycles during one month are compared with those observed on 223 megacycles. The visibility fades appear to be caused by scattering irregularities whose geographical distribution peaks near the maximum of the visual auroral zone. There is evidence of contributions from both the E-layer and F-layer. Primary occurrence maxima appear in autumn and at magnetic midnight.