A study of the Jovian "energetic magnetospheric events" observed by Galileo: role in the radial plasma transport

Using the Galileo Plasma Wave Subsystem (PWS) experiment, we analyze the large-scale energetic events that recurrently occur in the Jovian magnetosphere. As described by Louarn et al. [1998] these sporadic phenomena are associated with enhancements in the flux of the various auroral radio emissions, with the creation of new sources of radiation in the Io torus, and correspond to large fluctuations in the magnetodisc density. These events have been interpreted as sudden releases of energy in the Jovian magnetosphere. In order to better characterize them Eve study an extended PWS data set (orbits G2, G7, and G8) corresponding to 130 days of observations during which 32 energetic events have been unambiguously identified. We conclude the following: (1) The periods of enhanced energy releases In the magnetosphere (as indicated by increases in the auroral radio flux) are almost systematically initiated by an energetic event. (2) The periodicity of the events changes from one orbit to the other and it is shown that the more frequent they are, the denser is the plasma sheet. (3) In a large majority of cases the events occur as the disc is thin and relatively depleted in plasma. A few hours after the events, a thick and heavily populated magnetodisc is observed. It then thins, and its density progressively decreases over a timescale of a few tens of hours. Our interpretation is that the events correspond to sequences of rapid plasma loading of the magnetodisc that are followed by much more progressive evacuations of the magnetodisc plasma. The global plasma content of the magnetodisc would thus increase with their frequency. This study suggests that the energetic events are related to an instability developing in the external part of the Io torus or in the close magnetodisc that sporadically injects new plasma populations in the more distant magnetodisc. The associated transport process appears to be efficient enough to explain the outward evacuation of a significant fraction of the plasma created at the Io orbit (a few 10(28) ions/s).