Energetic particle bursts in the predawn Jovian magnetotail

Abstract

From September to October 1996 the Galileo spacecraft crossed through the distant predawn tail region of the Jovian magnetosphere. The Energetic Particles Detector (EPD) onboard Galileo recorded a series of energetic particle flow bursts in the region beyond 80 RJ to the apojove at 113 RJ. The events are similar in nature to an event observed with the hot plasma instrument (LECP) onboard Voyager 2. The individual events last for several hours and cover the whole energy range from 15 keV to 55 MeV. The majority of them show considerable intensity increases which are most prominent for heavy ions. The events exhibit high radially outward directed anisotropies suggesting strongly collimated radial outflowing ion beams. The Voyager event was observed beyond the corotation boundary within a magnetospheric boundary layer termed the magnetospheric wind region and consequently it was assumed that the underlying process is connected with a boundary layer instability. However, the Galileo observations show the bursts being embedded in a general corotation flow. It is thus suggested that the flow bursts are driven by an internal plasma sheet process possibly associated with a major re‐configuration of the Jovian magnetotail. A series of five very prominent flow bursts observed near apojove of the orbit occurred quasi‐periodically with a repetition period of about 2.6±0.2 days which is presumably related to an internal time constant of the Jovian magnetotail.