Disturbances in the lower ionosphere observed at VHF following the solar flare of 23 February 1956 with particular reference to auroral-zone absorption

Abstract

Observations at the time of the great solar flare of 23 February 1956 of oblique-incidence signal intensities and simultaneous observations of the background cosmic noise were made at VHF for a number of high-latitude communication links employing the ionospheric-scatter mode of propagation. During the flare and for some hours afterward, all paths lay in the dark hemisphere. Virtually synchronously with the arrival of solar cosmic rays, a sharp signal-intensity enhancement was observed, which is tentatively explainable if it is supposed that the first-arriving solar cosmic rays were predominantly of positive charge. The unusually stable night-time absorption which developed in one to three hours after the flare and the much greater following daytime absorption are explained in some detail as consequences of the deposition in the D region of moderately heavy solar atomic ions, such as calcium, having ionization potentials low compared with the normal atmospheric constituents. The absorption effects, which were limited to fairly high geomagnetic latitudes, gradually died away over a period of several days. The absence of significant magnetic disturbance and unusual auroral activity for nearly 48 hours after the flare is shown to be in accord with the suggested explanation of the absorption effects. Asymmetry about local noon was observed in the absorption effects on signal intensity, for which an explanation is suggested.