Increases in intensity of solar cosmic rays before sudden commencements of geomagnetic storms

Abstract

On at least three occasions, marked enhancement has been observed in the intensity of polar-cap absorption of radio waves before sudden commencement of a geomagnetic storm. During the most recent of the events, on September 30, 1961, equipment on board the satellites Explorer 12 and Injun 1 confirmed an earlier suggestion that the enhanced absorption is due to an increase in the flux of low-energy solar cosmic rays. The available evidence about the pre-SC enhancements is presented in detail, and the effect is interpreted in terms of a compression of the particles between two shock waves in the interplanetary medium. One of these shock waves, associated with the geomagnetic storm, approaches the earth with a speed of the order of 1000 km/sec, whereas the other is a ‘standing’ shock wave set up by the motion of the solar wind past the magnetosphere. The net effect of the compression is to cause the particles to crowd together and to increase their energy by Fermi acceleration. Both effects tend to increase the flux above a given energy in the vicinity of the earth. It is shown that many of the observed features can be explained by this interpretation.