VLF observations at college, Alaska, of various D-region disturbance phenomena

Abstract

Between late 1961 and mid-1964 several stabilized VLF transmitters have been monitored at College, Alaska. During that period 1846 optically detected solar flares were observed when our NBA recorder was operating and the path was completely sunlit. Of these, 66 produced phase anomalies on our NBA records. The probability of occurrence increased approximately linearly with flare area, but was less than probabilities for SID's determined by other methods. This indicates the VLF phase is not as sensitive as other SID indicators. The SPA decay was very nearly exponential on the College signals. The decay was significantly faster at 10.2 kc/s than at 20 kc/s frequencies. Polar cap precipitation events disrupt polar VLF propagation. The polar D-region was depressed significantly for periods of up to four or five days after relatively minor PCA events. Immediately after three events the phase from GBR was advanced 180 to 270 degrees beyond normal daytime values. VLF phase anomalies produced by (1) the U.S. and U.S.S.R. high altitude nuclear explosions in 1962, (2) the 20 July 1963 solar eclipse, and (3) the 1 October 1961, sub-auroral-latitude particle precipitation event are shown and discussed.