Longitudinal control of Jovian magnetopause motion

The magnetopause crossings of the Pioneer 10 and 11 spacecraft in Jovian magnetic coordinates (system III) are largely restricted in longitude to one hemisphere of Jupiter. This hemisphere is the one that has been identified by Vasyliunas (1975) as the 'active hemisphere'. This finding is interpreted as indicating that the magnetopause of the active hemisphere moves inward and outward with a radial speed that is typically faster than that of the inactive hemisphere.