Cassini Plasma Spectrometer electron measurements close to the magnetopause of Jupiter

Abstract

During late 2000 and early 2001 the Cassini satellite made a flyby of Jupiter en route to Saturn. The flyby speed was about 20 km/s, and the trajectory brought the spacecraft through the magnetosheath on the afternoonside. The Cassini Plasma Spectrometer (CAPS) instrument, consisting of two ion sensors and an electron spectrometer, was thus able to measure the plasma throughout this region on an unprecedented timescale. The electron spectrometer measured electrons in the energy range of about 1 eV to 27 keV with up to 2 s spectral resolution. During January 2001, Cassini twice entered into the Jovian magnetosphere, and the electron measurements from these periods will be presented. From these observations it is clear the Jovian magnetopause is effectively a firm boundary as far as low‐energy plasma is concerned. On the other hand, high‐energy particles (in the MeV range, measured by the Magnetospheric Imaging Instrument (MIMI)) were observed both inside and outside of the magnetopause. Inside the magnetosphere the spacecraft was charged to values of +50 V or so for most of the time, indicative of a cold and tenuous plasma. However, for about an hour of observation time, another magnetospheric electron population was also seen. This population had a density in the range 104 m−3 to 105 m−3 and a corresponding temperature on the order of 30 eV or so. Hence these measurements are distinct from both other ELS observations inside the magnetosphere as well as from the observations carried out in the magnetosheath.