The 1.3-year variation in solar wind speed and geomagnetic activity

Recent studies have discovered a strong 1.3-year variation in solar wind speed. It has been shown that this variation occurs concurrently at different heliocentric distances around the ecliptic. The same periodicity has also been observed in geomagnetic activity and occurrence of aurorae which are greatly dependent on solar wind speed. We study this periodicity using solar wind speed measurements at 1 AU from 1964 onward, and the Kp index of geomagnetic activity from 1932 onward. We show that the 1.3-year variation is a quasi-periodicity which occurs during even solar cycles. On the other hand, during odd cycles, we find a somewhat longer periodicity with a period varying from 1.5-1.7 years. Both of these periodicities are expected to be due to the evolution of coronal holes. Therefore, the observed difference in period implies a difference in the evolution of coronal holes during even and odd cycles. (C) 2000 COSPAR. Published by Elsevier Science Ltd.