Geomagnetic Activity and Size of Magnetospheric Cavity

The magnetopause locations as determined by the Explorer 12 SpL charged-particle detector and the Imp I plasma experiment can be used to study the relation between the solar wind parameters and geomagnetic activity. As will be seen, no strong correlation is evident between the size of the magnetosphere, as determined from magnetopause crossings, and geomagnetic activity indexes. It is argued that this finding supports the thesis that time variations in solar wind parameters are the principal agent responsible for the geomagnetic activity characterized by a• or K• indexes [Dessler, 1962; Akaso•u and Chapman, 1963; Dessler and Fejer, 1963; Dessler and Walters, 1964; Akaso•u, 1965]. The Explorer 12 charged-particle SpL detector was sensitive to electrons in the energy range 40 kev _• E _• 50 kev. Boundaries were observed beyond which the SpL count rate fell from a significant counting rate to cosmicray background levels. Freeman [1964] identified these boundaries as magnetopause crossings. The location of the magnetopause has been determined from the Imp i plasma probe experiment by means of the following criterion [Wolfe et al., 1966]' Inside the magnetosphere, no plasma flux was observed in the energy range of 600 ev to 3740 ev. The radial distance R at which a plasma flux in the above energy range started to appear is taken as the position of the magnetospheric boundary or the magnetopause. Beyond this distance Imp I is considered to be within the magnetosheath behind the bow shock. A list of magnetopause distances is given in the paper by Wolfe et al. [1966]. A total of 87 magnetopause distances are used from the two satellites' 44 crossing distances from Explorer 12 covering 1330 to 0700 hours local time and 43 crossing distances from Imp I covering 1140 to 0400 hours local time. For purposes of comparison we have reduced these boundary distances to the equivalent distances that would be obtained if all measurements were made at the stagnation point, i.e., the point where the plasma flow velocity tangential to the magnetopause is zero. The following two corrections are made. First, a latitude correction is made following Ness et al. [1964]. The boundary distances R are corrected by a factor (1 + 3 sin' X,) TM, where X, is geomagnetic latitude of the subsolar point, to give R,, the magnetopause distance in the ecliptic plane. These corrections to R due to the variation of the geomagnetic latitude of the subsolar point amount to 12% at most. The values of R, correspond to the geometry in which the solar wind is normally incident on the axis of the earth's magnetic dipole. Secondly, a longitudinal correction is applied to further reduce all R values to the corresponding values, Rm, that would be observed at 1200 hour local time. To make this correction we have used the magnetospheric boundary traces calculated by Mead and Beard [1964], which have been shown to be a good representation of the geometry of the front portion of the magnetosphere [Ness et al., 1964]. From these traces we can obtain the ratio R,/R, with which to obtain Rm, the value of R normalized to the equatorial plane and 1200 hours local time. •