HIGH-FREQUENCY RADIO-WAVE BLACK-OUTS AT MEDIUM AND HIGH LATITUDES DURING A SOLAR CYCLE

Recent studies at the Defence Research Telecommunications Establishment, Ottawa, comparing v.h.f. riometer and h.f. ionosonde data for the International Geophysical Year, have shown that the two types of absorption events, 'polar cap' and 'auroral', can be identified in the occurrence patterns of ionosonde black-outs. A study based on this comparison has been made of all black-outs observed at a number of medium- and high-latitude ionosonde stations during the period 1949 to 1959. It has been found that the two kinds of absorption events show markedly different variations during the 11 years. Several other temporal and spatial features of the phenomena are also discussed.