Physics – Plasma Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufmsm14a..02s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #SM14A-02
Physics
Plasma Physics
[6275] Planetary Sciences: Solar System Objects / Saturn, [7807] Space Plasma Physics / Charged Particle Motion And Acceleration, [7845] Space Plasma Physics / Particle Acceleration, [7846] Space Plasma Physics / Plasma Energization
Scientific paper
The energetic particle population in Saturn's magnetosphere was initially sampled during the Pioneer 11 and Voyager 1 and 2 flybys in the early 1980s. It was, however, the far more sophisticated energetic particle suite, the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft that offered new insight of the energetic particles in Saturn's environment. Since July 2004, the three energetic particle detectors of MIMI, the Low Energy Magnetospheric Measurement System (LEMMS), the Charge Energy Mass Spectrometer (CHEMS) and the Ion and Neutral Camera (INCA), provide energetic ion directional intensities, ion and electron energy spectra and ion composition in a keV-to-MeV energy range. In particular, through detailed energetic neutral atoms (ENA) imaging, INCA resolved the perennial limitation of in situ data (spatial vs. temporal variability), offering an overview of large parts of the magnetosphere and capturing the ongoing dynamical activity (e.g. hot plasma injections), regardless of the spacecraft's position. The results obtained so far have clearly revealed that hot plasma plays a key role in several processes active in a wide range of spatial scales in the Saturnian magnetosphere, such as the formation of high energy trapped particle radiation belts in the inner magnetosphere and of a partial, rotating ring current in the middle and outer magnetosphere, the plasma energization in the midnight-to-dawn local time sector and the variability of the Saturnian auroral UV and radio emissions. The extended coverage provided by the numerous (over 150 as of August 2011) revolutions of Cassini has helped us construct a comprehensive (yet not complete) picture of the hot plasma distribution and composition in Saturn's magnetosphere. The most surprising characteristic was the direct observation that the energetic ion distribution is strongly asymmetric with local time, forming a broadened dayside plasma sheet which becomes thinner and more intense in the nightside, with a seasonal, solar wind-driven tilt. Comparison with thermal plasma data (Cassini/CAPS) showed that at least 50% of the total plasma pressure at larger (>8 Rs) radial distances is contributed by the hot (>keV) plasma, with energetic O+ ions being the major pressure carriers. The inclusion of magnetic field data (Cassini/MAG) verified, more than three decades after the Voyager flybys, that Saturn possesses a high plasma β magnetosphere (β>1 beyond 8 Rs). In addition, in both in situ and remote measurements, the energetic particle populations exhibit intense temporal (non-periodic) variability, with typical range of at least one order of magnitude. In this paper we review the most significant energetic particle-related Cassini findings, and discus some of the scientific issues of great interest and importance that still need to be addressed during the remaining years of the mission, such as the well observed -still not fully explained- periodicities in plasma and energetic particle properties, the seasonal dependence of its distribution and better understanding of the physical mechanism causing the hot plasma injections.
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