Secular Movement of the Zone of Auroral Blackout

Abstract

THE magnetic dip pole in 1922 and 1942 was close to the position 71° N. 97° W. whereas in 1955 and 1965 it was near 76° N. 101 ° W.¹. A test has been made to find whether this movement was accompanied by changes in the rate of occurrence of radio blackout near Inverness (57.5° N., 4° W.) using the f_min tables from the vertical incidence data taken in this area between 1942 and 1963. The ionograms are reasonably comparable for the period. Data for months on which the number of blackout entries (B) were abnormally large because of low critical frequencies at night were rejected. Independent studies were made for epochs near solar minimum, 1942–43 and 1962–63, and near solar maximum, 1946–47 and 1957–58, and significant differences in the numbers of days with blackout were found for both sets of data.