O.E. Malandraki1, S.M. Krimigis1,2, E.T. Sarris3, N. Sergis1, K. Dialynas1, D.G. Mitchell2, D.C. Hamilton4
1Office for Space
Research and Technology, Academy of Athens, Athens, Greece
2Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland, USA
3Democritus University of Thrace, Xanthi, Greece
4University of Maryland, Department of Physics and Astronomy, MD, USA
We will present unique observations obtained by the Magnetospheric Imaging Instrument (MIMI) on the Cassini spacecraft of the energetic ion population in the environment upstream from the noon-to-predawn side of the Kronian magnetosphere during the approach phase and subsequent several orbits of the Cassini spacecraft around the planet. High sensitivity observations of energetic ion directional intensities, energy spectra, and ion composition were obtained by the Ion and Neutral Camera (INCA) of the MIMI instrument complement with a geometry factor of ~ 2.5 cm2 sr and some capability of separating light (H, He) and heavier (C, N, O) ion groups (henceforth referred to as 'hydrogen' and 'oxygen' respectively). Charge state information was provided by the Charge-Energy-Mass-Spectrometer (CHEMS) over the range ~3 to 220 keV per charge. The observations revealed the presence of distinct upstream bursts of energetic hydrogen and oxygen ions up to distances of 135 RS. The events exhibited the following characteristics: (1) The hydrogen ion bursts are observed in the energy range 3 to -220 keV (and occasionally to E > 220 keV) and the oxygen ion bursts in the energy range 32 to -300 keV. (2) The duration of the ion bursts is several minutes up to 4 hrs. (3) The events are of varying composition, with some exhibiting significant fluxes of oxygen and other magnetospheric species. (4) The bursts have a filamentary structure with some exhibiting distinct signatures of 'velocity-filtering effects' at the edges of convecting IMF filaments. (5) Some ion bursts are accompanied by distinct diamagnetic field depressions and exhibit wave structures consistent with ion cyclotron waves for both hydrogen and oxygen. Given that energetic ions trapped within the magnetosphere of Saturn are mostly H+ and O+ (Krimigis et al, 2005), we conclude that O+-rich upstream events must be particles leaking from Saturn's magnetosphere under favorable IMF conditions. The implications for the Cassini/MIMI observations of recent findings for lower energy ion upstream events by the Cassini/CAPS instrument will be discussed. We will also present in detail the spectral features of the upstream events and results on their anisotropy characteristics. The theoretical implications of the observations will be addressed and similarity of such events to those observed at other planets will also be discussed.
Reference: Krimigis et al, Dynamics of Saturns magnetosphere from MIMI during Cassini's orbital insertion, Science, 307, 1270-1273, 2005.