Very-low-frequency effects from the November 10, 1961, polar-cap absorption event

A weak polar-cap absorption event began on November 10, 1961. It profoundly affected the polar Rugby, UK, to College, Alaska, VLF path. Twenty minutes after the observation of the solar flare believed responsible, the 16.0-kc/s GBR signal shifted phase approximately 250° and amplitude −20 db over a 1-hour period. During the event the diurnal variations of phase and amplitude increased in magnitude and changed markedly from normal patterns. The effective height of the VLF wave guide over the polar cap dropped to about 5 km below the normal D-region height even though much of the path was completely dark. Over most of the path the reflection height was below that for the normal, sunlit D region, but the signal amplitude was significantly greater than that when the entire path was sunlit. The proton bombardment apparently lasted for at least 5 days after the onset of the event.