Evidence from charged particle studies for the distortion of the Jovian magnetosphere

Abstract

In this paper we consider the relationship between the rotation of Jupiter's magnetic field and time variations in the intensity of ∼6- to 30-MeV electrons observed by the University of Chicago experiment on Pioneer 10 in the outer regions of Jupiter's magnetosphere (R ⪞ 20 RJ). For R ⪝ 40 RJ we find our observations to be consistent with rigid corotation of the magnetosphere with Jupiter. For R ⪞ 40 RJ, significant deviations from rigid corotation appear with the observed phase of the intensity variations leading the phase expected for rigid corotation on the inbound pass and lagging on the outbound pass. From a different point of view we find that the time delay between the observed times of intensity minimums and the times expected on the basis of a rigid 9 hour 55 minute period for the intensity variations increased steadily while Pioneer 10 was within the magnetosphere and had reached approximately a 10-hour time difference when the spacecraft left the magnetosphere at R ≅ 98 RJ outbound. We discuss the alternative hypotheses that in the far outer regions of Jupiter's magnetosphere (R ≅ 90 RJ) these deviations are the result of magnetospheric distortions arising from forces exerted by the solar wind and from drag due to lack of corotation of the magnetospheric plasma at large radial distances from the planet or that the particle intensity in the outer magnetosphere is independent of magnetic latitude and system III longitude but is a function only of time. Although the latter hypothesis is more consistent with observations of Jovian electrons in interplanetary space reported by D. L. Chenette et al. (1974), the observations inside Jupiter's magnetosphere reported here seem to favor particle confinement to an extended magnetic equatorial region, significant distortions of the magnetosphere being invoked to explain the changes in phase of the intensity variations.