Compression of Jupiter's magnetosphere by the solar wind

Abstract

A study of the major changes in the solar wind during the Pioneer 10 and 11 encounters and their influence on the size of the Jovian magnetosphere is reported. Simultaneous sets of encounter data acquired by the Jet Propulsion Laboratory vector helium magnetometer, the Ames Research Center plasma analyzer, and the University of California at San Diego trapped radiation detector have been compared with data acquired simultaneously in interplanetary space by the sister spacecraft. Of particular interest were four intervals during which it appeared that the spacecraft had reentered the magnetosheath near 50 RJ after having first entered the magnetosphere near 100 RJ. The principal outcome of the study is that in three of these cases the reentries into the magnetosheath occurred when high‐speed solar wind streams and their associated interaction regions were expected to arrive at Jupiter. Thus the study supports the hypothesis advanced previously that the Jovian magnetosphere had undergone a large‐scale compression. The results are contrary to an alternative hypothesis that the Pioneers had traversed a spatial region located inside the magnetosphere possibly associated with plasma outflow. The fourth case, which was observed by Pioneer 11 outbound, appears to have occurred during quiet interplanetary conditions. However, a detailed reinvestigation of magnetic field and plasma data during this interval shows that the spacecraft had reentered the magnetosheath and not a region interior to the magnetosphere. The reentry into the magnetosheath and the subsequent return to the magnetosphere were separated by an interval of 10 hours and would have been expected to occur when the spacecraft was at its highest magnetic latitude. It is concluded, tentatively, that this reentry was the result of a large‐scale north‐south motion intrinsic to the Jovian magnetosphere. The question of whether or not the magnetic field just inside the magnetopause is sufficiently strong to withstand the pressure of the incident solar wind has been reexamined within the context of this present study. The field appears able to hold off the solar wind both at 100 RJ and near 50 RJ. The compressibility of the Jovian magnetosphere is enhanced because the field inside the magnetopause is not the planetary field but is principally caused by currents inside the magnetosphere, presumably the equatorial current sheet. The possible acceleration of energetic trapped radiation when the magnetosphere was compressed has been investigated. Comparison of the increased particle fluxes and the magnetic field shows that gyrobetatron acceleration can be discounted. Based on the measured time difference between the particle enhancement and the arrival of the magnetopause at the spacecraft, an estimate is derived for the average plasma density inside the magnetosphere of 1–10 cm−3. Finally, the characteristic time constants appropriate to an electric circuit model of Jupiter's magnetosphere have been estimated as being in the range between 15 and 50 hours.