Ring current at Saturn: Energetic particle pressure in Saturn's equatorial magnetosphere measured with Cassini/MIMI

The Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft provides measurements of the energetic ion population within the magnetosphere of Saturn. Energetic ion directional intensities, energy spectra and ion composition, are measured by the Charge Energy Mass Spectrometer (CHEMS) over the range similar to 3 to 236 keV per charge and by the Low Energy Magnetospheric Measurements System (LEMMS) for ions in the range 0.024 < E < 18 MeV. This work reports preliminary results of partial particle pressure distributions throughout the equatorial magnetosphere and comparison with in situ measurements of the magnetic pressure provided by Cassini's magnetometer. The results cover 11 passes from late 2005 to early 2006, when the spacecraft was particularly close to the nominal magnetic equator in the range 5 < R < 20 R-S and can be summarized as follows: (1) the plasma beta (particle pressure/magnetic pressure) profile increases radially outward to maximum values of >= 1 at L > 10 RS; (2) most particle pressure is contained in the range of 10 < E < 150 keV; and (3) in the high beta region 10 < L < 19, where the apparent ring current resides, oxygen generally contributes more than 50% of the total particle pressure. The results demonstrate that typical assumptions of MHD models, whereby particle pressure is presumed to reside with the cold plasma, are not supported by the data.