Jovian electron propagation in three dimensions of the heliosphere: The Ulysses Investigations

Abstract

We report investigations of Jovian relativistic electrons in the interplanetary medium that provide new insights into both the physical processes by which the Jovian magnetosphere releases its trapped, relativistic electrons into the interplanetary medium, and the modes of their interplanetary propagation. These studies were dependent on the unique postencounter trajectory for Ulysses. The spacecraft remained close to the radial distance of Jupiter (∼5.2 AU) and moved southward on the duskside by only ∼12° in heliographic latitude and <8° in heliographic azimuth relative to Jupiter for the period of ∼100° days of this study. During this period the nominal Parker spiral interplanetary magnetic field with its alternating polarities sector structure established direct magnetic field line connections frequently between Jupiter and the spacecraft. These unique conditions made it possible to investigate in detail, for approximately four solar rotations, both the Jovian electron burst phenomenon and the continuous, diffusive interplanetary propagation of Jovian electrons. The Jovian electron burst phenomenon in the energy range 1‐20 MeV, with flux increasing up to 50 times the diffusive Jovian electron flux are now discovered to occur in both multiples of the planetary rotation period (∼10 hours) and occasionally in half rotation periods, namely, in sequences of 5‐, 10‐, 20‐, or 40‐hour intervals. Electrons in the burst are shown to stream from the magnetosphere and not the magnetotail to the spacecraft during times when the interplanetary magnetic field vector direction includes both Jupiter and the spacecraft. Over 90% of all electron bursts lasting ≥6 hours occurred during the positive magnetic field sector. The few bursts occurring in a negative sector (<10%) were associated with rapid changes in magnetic field orientation near a sector boundary. We found that the time for these burst sequences corresponded to either the dawnside or the duskside tilt of the magnetic dipole. This dawn‐dusk dipole tilt is in a plane approximately orthogonal to the Sun‐Jupiter line and aligned with the average interplanetary magnetic field direction at ∼5 AU. To interpret these measurements, we assumed that the release mechanism is intermittent reconnection of the interplanetary magnetic field with the opposite polarity of a component of the polar magnetospheric field. These experimental results appeared to exclude some models for electron release such as the magnetic anomaly model where the electrons are released into the magnetotail. The diffusive Jovian electron flux continuously present in the interplanetary medium confirmed the “10‐hour clock” variations of the differential energy spectral index found by Pioneer 10 in 1973. Present evidence indicates that this continuum of Jovian electrons with its 10‐hour rocking spectrum may be associated with the same release mechanism as the electron bursts.