Ionospheric disturbances following a solar flare

Abstract

Northern hemispheric charts of ƒmin, ƒoEs, and ƒoF2 at intervals of 3 hours have been constructed for the disturbed period from September 12 through 15, 1957. Enhanced ƒmin values were found over the polar cap on September 12. By the time of the sudden commencement (SC) of geomagnetic activity (0046Z, September 13) the ƒmin disturbance spread outward to the auroral zone. Following the SC, the ƒmin disturbance formed in an elliptical ring around the earth while the polar-cap ƒmin disturbance dissipated. The elliptical ring (coincident with constant dip lines) then expanded southward, and by 0900Z, the approximate time of the main phase, the ƒmin ring disturbance reached a constant dip of about 72°. Just to the north of this ring, in Canada, a crescent-shaped sporadic-E disturbance developed. It also moved southward. Subsequently, a band of sporadic E appeared over northern Asia. The ƒoF2 disturbance began about the time of the SC and lasted about 36 hours. On the basis of the ƒmin and ƒoEs patterns, it is concluded that, prior to the SC, solar corpuscles impinge on the lower ionosphere directly from outside the earth's ‘magnetosphere’ The post-SC ƒmin and ƒoEs disturbances are attributed to trapped radiation drifting at relatively low altitudes (below about 2000 km). The southward expansion of the disturbances is a result of the strengthening of the solar stream.