Energetic particle observations in the vicinity of Jupiter: Cassini MIMI/LEMMS results

Abstract

We report on energetic particle measurements from the Low Energy Magnetospheric Measurement System (LEMMS) on board the Cassini spacecraft during the Jupiter flyby (October 2000 to April 2001). Cassini passed Jupiter on its way to Saturn on the dusk flank of the magnetosphere and explored for the first time the dusk‐to‐midnight magnetosheath of the planet. The flyby period can be divided into three distinct regimes where energetic particle parameter changes are observed. The first period (October 2000 to 10 January 2001) covers the approach phase toward the planet, including bow shock crossings in and out of the magnetosheath as well as the closest approach. In this period, LEMMS responded on solar wind pressure pulses and recorded the passing of two interplanetary shocks. Hours before the first bow shock crossing, low‐energy particle intensity increases were observed, possibly with Jovian origin. Before the closest approach approach to the planet two inbound and one outbound bow shock crossings have been observed. The second period (9–10 January) is the period where Cassini briefly entered the magnetosphere twice at about 200 RJ. LEMMS data are only available during a large portion of the second encounter. Inside the magnetosphere the energetic electrons showed a bidirectional pitch angle distribution along the magnetic field, indicative of a closed magnetic field configuration predominantly in the north‐south direction. In addition, quasi‐periodic electron intensity variations with periods of ≈40 min were observed. The third period (January to April 2001) covers the distances between 205 and 800 RJ. The spacecraft was skimming along the bow shock boundary in the dusk‐to‐midnight sector. More than 44 inbound and outbound bow shock crossings have been observed. The most interesting magnetosheath encounter occurred between day 19 and day 28 at distances between 300 and 400 RJ. In that period, LEMMS observed strong intensification of MeV‐electron fluxes to values similar to those measured inside the magnetosphere as reported by Krupp et al. [2002]. Most of these increases in electrons were associated with sign changes in the north‐south component of the magnetic field. We interpret these observations in terms of magnetic dayside reconnection and particle leakage through the duskside magnetosheath.