Johns Hopkins Applied Physics Laboratory
Adventures at Saturn – New results from the Cassini/MIMI instrument
The Cassini spacecraft began orbit around Saturn in July 2004 and its various instruments have been sending back data continuously since then. In its four years of observations, the Magnetospheric Imaging Instrument (MIMI) on Cassini has made several discoveries about Saturn’s magnetosphere. The Low Energy Magnetospheric Measurement System (LEMMS) and the Charge Energy Mass Spectrometer (CHEMS), both subsystems of MIMI, have characterized the charged particle environment from ~2 RS (1 RS= 60268 km) to ~60 RS. Wavelet and Lomb- Scargle periodogram analyses of the charged particles observed by LEMMS and CHEMS have revealed strong and persistent periodicities in all species at 10.8 hours, which is close to the 10.793-hour period of Saturn’s kilometric radiation (SKR). In Saturn’s outer magnetosphere, the charged particles fluxes seem to organize into a one-armed spiral pattern when viewed in a longitude system rotating with the SKR period. The base of the spiral appears connected to an outflow region postulated by plasma and radio wave observations. A third subsystem of MIMI, the Ion-Neutral Camera (INCA) has measured energetic neutral atoms (ENA) generated by the charge-exchange reactions between energetic ions and the cold gas between ~3 RS and ~15 RS around Saturn. “Blobs” in the neutral atom distribution rotate at speeds much greater than Keplerian speeds but have a speed distribution that changes abruptly near the orbit of one of Saturn’s moons. Statistically, the neutral atom emissions are strongest on the night side of the planet, suggesting that substorm-like processes may cause injection of energetic ions from the magnetotail. However, because the ENA emissions are continuous, the injections must be driven by dynamical processes internal to the magnetosphere, rather than externally by the solar wind. The “cam” or magnetic anomaly model of a feature co-rotating with Saturn appears to describe all these phenomena, although the ultimate source of the feature remains unknown.