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Science 25 February 2005:
Vol. 307. no. 5713, pp. 1270 - 1273
DOI: 10.1126/science.1105978
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Reports

Dynamics of Saturn's Magnetosphere from MIMI During Cassini's Orbital Insertion

S. M. Krimigis,1* D. G. Mitchell,1 D. C. Hamilton,2 N. Krupp,3 S. Livi,1 E. C. Roelof,1 J. Dandouras,4 T. P. Armstrong,5 B. H. Mauk,1 C. Paranicas,1 P. C. Brandt,1 S. Bolton,6 A. F. Cheng,1 T. Choo,1 G. Gloeckler,2 J. Hayes,1 K. C. Hsieh,7 W.-H. Ip,8 S. Jaskulek,1 E. P. Keath,1 E. Kirsch,3 M. Kusterer,1 A. Lagg,3 L. J. Lanzerotti,9,10 D. LaVallee,1 J. Manweiler,5 R. W. McEntire,1 W. Rasmuss,5 J. Saur,1 F. S. Turner,1 D. J. Williams,1 J. Woch3

The Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft observed the saturnian magnetosphere from January 2004 until Saturn orbit insertion (SOI) on 1 July 2004. The MIMI sensors observed frequent energetic particle activity in interplanetary space for several months before SOI. When the imaging sensor was switched to its energetic neutral atom (ENA) operating mode on 20 February 2004, at ~103 times Saturn's radius RS (0.43 astronomical units), a weak but persistent signal was observed from the magnetosphere. About 10 days before SOI, the magnetosphere exhibited a day-night asymmetry that varied with an ~11-hour periodicity. Once Cassini entered the magnetosphere, in situ measurements showed high concentrations of H+, H2+, O+, OH+, and H2O+ and low concentrations of N+. The radial dependence of ion intensity profiles implies neutral gas densities sufficient to produce high loss rates of trapped ions from the middle and inner magnetosphere. ENA imaging has revealed a radiation belt that resides inward of the D ring and is probably the result of double charge exchange between the main radiation belt and the upper layers of Saturn's exosphere.

1 Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA.
2 Department of Physics, University of Maryland, College Park, MD 20742, USA.
3 Max-Planck-Institut für Sonnensystemforschung, D-37191 Lindau, Germany.
4 Centre D'Etude Spatiale Des Rayonnements, F-31028 Toulouse, France.
5 Fundamental Technologies Inc., Lawrence, KS 66049, USA.
6 Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
7 Department of Physics, University of Arizona, Tucson, AZ 85721, USA.
8 Graduate Institute of Astronomy, National Central University, Jhongli City, Republic of China.
9 Bell Laboratories, Murray Hill, NJ 07974, USA.
10 New Jersey Institute of Technology, Newark, NJ 07102, USA.

* To whom correspondence should be addressed. E-mail: tom.krimigis{at}jhuapl.edu

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