A study of solar activity associated with polar-cap absorption

Abstract

Systematic examination of all unusual solar radio and optical emission before the onset of 43 polar-cap absorption events shows which type of solar activity most frequently precedes PCA. This provides an objective basis for assignment of the solar event most probably associated with each of the individual PCA events. Relations among various characteristics of solar and PCA events are examined. These data present little evidence for association of PCA with solar flares that occur on the far side of the sun, nor do they indicate any significant dependence of occurrence or of delay time of PCA on position of the associated flare. Delay from flare to absorption varies from values of several minutes, appropriate to the energy of the solar particles, up to several hours. A definite relation is found between delay time and phase of the solar cycle, with long delay times near the time of maximum solar activity.