Annual and daily variation in the occurrence of positive Dst index

Summary. The frequency of occurrence of the positive D_st for each UT hour has been computed using data for the years 1957–77. For all hours the seasonal variation shows a peak in June. The annual variation conforms with the Malin-Isikara mechanism. While the annual component of D_st+ exhibits a significant UT dependence which cannot be explained by the McIntosh effect, the semi-annual component has a relatively smaller significant time dependence. The diurnal component of D_st+ has a pronounced annual variation with a June maximum. The results presented here are compared and discussed in the light of an earlier analysis of D_st by Mayaud.