Jovian modulation of interplanetary electrons as observed with Voyagers 1 and 2

The release of magnetospheric electrons from Jupiter into interplanetary space is modulated by the Jovian rotation period. This effect was initially discovered by Pioneer 10, and the Voyager 1 and 2 observations permit a more detailed study of this modulation. It was found that the modulation period agrees on the average with the synodic period of Jupiter (9 h 55 min 33.12 s), but over intervals of weeks it can differ from the synodic period by several minutes. The lack of exact synchronization is attributed to changes of the plasma population in the Jovian magnetosphere. Such changes affect the magnetic field sweep‐back and departure from exact corotation. However, the magnetospheric asymmetry, which is responsible for the modulation, is always re‐established at the same longitude. Thus no long term departures occur from the synodic period. The Jovian modulation appears to be a persistent feature of the interaction between the solar wind and the magnetosphere, and the disappearance of the modulation away from Jupiter is attributed to interplanetary propagation conditions. Although modulation could be detected in interplanetary space out to only ∼10 8 km from Jupiter, it was still detectable at 3.8 AU behind Jupiter in the far magnetotail. This requires a mean free path in the tail ≥0.75 AU and good field connection along the tail to Jupiter. During two intervals, the electron spectrum was softer than the Jovian spectrum and was not modulated by the Jovian period. The time histories of these latter increases are similar to that of 30‐ to 60‐MeV protons measured simultaneously by the same detector system. They are associated with large solar cosmic ray events at 1 AU and appear to be of solar origin.