Increase of ionization associated with geomagnetic sudden commencements

Abstract

It has been suggested in previous studies on geomagnetic sudden commencements that charged particles from outside the earth's atmosphere seem to penetrate into the lower ionosphere at high latitudes and cause ionization there at the moment of sudden commencements [Matsushita, 1957; 1960]. Riometer records of 27.6 Mc/s cosmic noise obtained at Fort Yukon (geographic 66°34′N, 145°18′W), Barrow (geographic 71°31′N, 156°20′W), and Thule (geographic 76°33′N, 68°50′W), which are published in the IGY Aurora (Instrumental) Report, No. 1, Geophysical Institute, University of Alaska, clearly support the above suggestion. As is seen in Figure 1, an absorption of 0.1 to 5 db often begins at the same time as the sudden commencement and lasts 5 to 10 minutes. The time and date of all obvious sudden commencements in 1958 (all those reported by more than 30 geomagnetic stations in the world) and the occurrence of the absorption phenomenon are listed in Table 1. The asterisk in column ‘PCA’ indicates that polar-cap absorption due to solar flares preceded the sudden commencement; and the PCA in column ‘Increase of Absorption’ means that polar-cap absorption is still continuing at the time of the sudden commencement. It can be noticed in Table i that the increases of absorption occur more frequently at Barrow and Fort Yukon than at Thule; in other words, more in the auroral zone than in the polar cap. No absorption increases were found at the time of the less obvious sudden commencements, which are reported by fewer than 30 geomagnetic stations in the world.