For the first time, two spacecraft, Galileo and Cassini, observed Jupiter's magnetosphere simultaneously for nearly half a year between October 2000 and March 2001. This provided an unprecedented opportunity to disentangle spatial and temporal aspects of the dynamics of the Jovian magnetosphere. In this paper we report new results on the source of the leakage of energetic particles (electrons with energy 15 keV to several MeV and ions with energy > 30 keV) from the dusk side of the magnetosphere. The dual spacecraft measurements show clearly that magnetospheric particles leak directly into the interplanetary medium from the closed magnetosphere, and are the source for the “upstream” particle events [ Baker et al., 1996 ; Zwickl et al., 1981 ; Krimigis, 1992 ; Haggerty and Armstrong, 1999 ; Anagnostopoulos et al., 1998 ] that have been reported from instruments during prior single spacecraft encounters with the planet. These events, consisting of high-energy particles of Jovian origin, have been observed throughout the heliosphere [ Baker and Van Allen, 1976 ] and their propagation has recently been modelled [ Fichtner et al., 2000 ; Ferreira et al., 2001 ]. Jupiter then is an important contributor to the interplanetary charged particle fluxes, especially within an astronomical unit of the planet.