The extended Saturnian neutral cloud as revealed by global ENA simulations using Cassini/MIMI measurements

摘要

We show that the neutral gas vertical distribution at Saturn must be ~~3-4 times more extended than previously thought for the >5 R_Sregions, while the neutral H distribution is consistent with H densities that reach up to ~~150/cm~3close to the orbit of Titan. We utilize a technique to retrieve the global neutral gas distribution in Saturn's magnetosphere, using energetic ion and energetic neutral atom (ENA) measurements, obtained by the Magnetospheric Imaging Instrument (MIMI) onboard the Cassini spacecraft. Our ENA measurements are consistent with a neutral cloud that consists of H _2O, OH, H, and O, while the overall shapes and densities numbers concerning the neutral gas distributions are constrained according to already existing models as well as recent observations. The neutral gas distribution at Saturn is determined by simulating a 24-55 keV hydrogen image of the Saturnian magnetosphere, measured by the Ion and Neutral Camera (INCA), averaged over the time period from 1 July 2004 to 23 August 2005. The ionic input of the model includes a proton distribution of combined Charge Energy Mass Spectrometer (CHEMS, 3-230 keV/e), Low Energy Magnetospheric Measurements System (LEMMS, 30.7 keV to 2.3 MeV), and INCA (5-300 keV) in situ measurements. These measurements cover several passes from 1 July 2004 to 10 April 2007, at various local times over the dipole L range 5< L <20R_S. A parameterized neutral gas distribution is changed until agreement between the simulated and average INCA image is obtained. Key Points We simulate global ENA images to retrieve the neutral gas distribution at Saturn We produce energetic ion equatorial distributions using in-situ observations The neutral gas vertical distribution is more extended than previously thought