Magnetosheath field, geomagnetic indexapand stability of magnetopause

Recent satellite observations show that the magnetosheath contains a magnetic field that varies from 5 to 60 gammas [Cahill and Amazeen, 1963; Ness et al., 1964]. The direction of the field sometimes remains steady for half an hour or more. Effects of this magnetosheath field, however small, may be important for certain magnetospheric phenomena. Several theories have been put forth in which it is proposed that the interplanetary magnetic field influences geomagnetic activity [Dungey, 1961, 1963; Dessler and Fejer, 1963; Dessler and Walters, 1964]. Two of the theories relate geomagnetic activity to field direction, while the third theory relates geomagnetic activity to field magnitude and rate of change of direction. In this note, we use Explorer 12 magnetic field observations to study the effect of the magnitude and direction of the field in the region immediately outside the magnetosphere (the magnetosheath) on fluctuations in the field at ground level as measured by the Kp or ap indexes.