Detecting the Enceladus Neutral Torus via Water Group Pick Up Ions

Details Detecting the Enceladus Neutral Torus via Water Group Pick Up Ions Detecting the Enceladus Neutral Torus via Water Group Pick Up Ions

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p52b..06t&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P52B-06

Astronomy and Astrophysics

Astrophysics

2756 Planetary Magnetospheres (5443, 5737, 6033), 6275 Saturn, 6280 Saturnian Satellites

Scientific paper

One of the major discoveries1 of Cassini to date is the south polar icy plume at Enceladus (R ~ 4 RS). Models2 predict that this plume may be a source of both the extended (2-8 RS) OH neutral cloud observed by the Hubble space telescope3, and a new feature, a narrow (~0.5 to 1.0 RS) neutral water group torus centered on the Enceladus orbit. As the corotating and magnetically confined thermal plasma (mostly water group ions) streams through the gravitationally bound water group neutrals, charge exchange between the ions and neutrals is expected4 to occur yielding slower ions subsequently "picked up" by Saturn's magnetic field. The phase space density of these ions should show characteristics of a ring velocity distribution within the source, combined with subsequent scattering into a shell and possible adiabatic cooling at larger radial distances. Therefore, via analysis of Cassini in situ ion counting data, it may be possible to indirectly detect the neutral Enceladus torus, confirming the predictions in (2). In this study, Cassini plasma spectrometer (CAPS) data for equatorial orbits with favorable viewing5 are analyzed. The radial distance range of about 3.5 to 6.5 RS is considered covering data across the Enceladus orbit. Assuming flow speeds near co-rotation as reported in (6) yields a modeled water group ion core that is subtracted from the measured data. The resulting residual ion counting data has velocity space signatures resembling pick up ions. The strongest source region is identified about the Enceladus orbit with radial extent at least 1 RS, in qualitative agreement with predictions. Peak phase space density of these ions is perpendicular to the magnetic field, resembling a ring, as expected within the source region. At larger radial distances (e.g. R = 6 RS), the ring signature has evolved to a shell and the expected adiabatic cooling due to transport from the source outward is observed. Similarly strong pick up ion sources are not observed near the orbits of either Tethys or Dione. 1.) Science, "Cassini at Enceladus", special section, 10 March 2006. 2.) Johnson, R.E. et al., The Astrophysical Journal, pg L137, 20 June 2006. 3.) Shemansky et al., Nature, 27 May 1993. 4.) Johnson, R.E., M. Liu, and E.C. Sittler, Jr., Geophys. Res. Letts., 32, 17 Dec 2005. 5.) Wilson, R.J. et al., this meeting. 6.) Sittler, E.C. et al., Geophys. Res. Letts., 32, 15 June 2005.

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