JOVIAN ELECTRON JETS IN INTERPLANETARY SPACE

The COSPIN/KET experiment onboard Ulysses has been monitoring the flux of approximately 3-20 MeV electrons in interplanetary space since the launch of Ulysses in October 1990. The origin of these electrons has been known for a long time to be the Jovian magnetosphere. Propagation models assuming interplanetary diffusion of these electrons in the id eal Parker magnetic field were successfully developed in the past. The average electron flux measured by our experiment agrees with these mo dels for most of the times before and after the Jovian flyby of Februa ry 1992, i.e. in and out of the ecliptic down to 28 S of heliographic latitude for the last data presented here (end of March 1993). However , in addition to this average flux level well accounted for by diffusi on in an ideal Parker field, we have found very short duration electro n events which we call ''jets'', characterized by: (i) a sharp increas e and decrease of flux; (ii) a spectrum identical to the electron spec trum in the Jovian magnetosphere ; and (iii) a strong first-order anis otropy. These jets only occur when the magnetic field at Ulysses lies close to the direction of Jupiter, and most of the time (86% of the ev ents) points outwards from Jupiter, i.e. has the same polarity after t he flyby as the Jovian dipole (North to South). These events are inter preted as crossings by Ulysses of magnetic flux tubes or sheets direct ly connected to the location of the Jovian magnetosphere from which el ectrons escape into interplanetary space. The average thickness of the se sheets is approximately 10(11) cm or approximately 14 Jovian radii. These jets are clearly identified up to 0.4 a.u. before the Jupiter f lyby in the ecliptic plane, and up to 0.9 a.u. out of the ecliptic. Mo reover, the characteristic rocking of the electron spectrum in the Jov ian magnetosphere with a 10 h periodicity is found to be present durin g the jets, and predominantly during them. In the past, this modulatio n has been reported to be present in interplanetary space as far as 1 a.u. upwind of Jupiter, a fact which cannot be accounted for by diffus ion in the average Parker magnetic field. Our finding gives a simple e xplanation to this phenomenon, the 10 h modulation being carried by th e ''jet'' electrons which travel with no appreciable diffusion along m agnetic field lines with a direction far from the ideal Parker spiral.