Neutral H2and H2+ions in the Saturnian magnetosphere

Abstract

[1] The Saturnian system is immersed in an extended cloud of neutrals. Although water vapor ejected from Enceladus' south pole is the dominant neutral source, photolysis and radiolysis of ices can release H2O, O2, and H2 from the icy ring particles and the icy satellites, and Titan's atmosphere is a source of H2. Once ionized, these neutrals are the source of the observed magnetospheric plasma. To understand the H2+ ion densities observed by the Cassini plasma spectrometer (CAPS), we developed a Monte Carlo test particle model to simulate the spatial morphology of the neutral H2 cloud and the resulting H2+ ion source rates. The H2 lifetime is constrained by its local chemistry, which is computed from the latest plasma measurements by Cassini CAPS data. The main rings, Enceladus' water torus, Rhea, and Titan are considered as the primary sources of H2 in our model. It is seen that H2 accumulates over Saturn's main rings because of thermal accommodation with the ring particles, and Titan is the dominant source of H2 in the outer magnetosphere (>∼6 RS). From ∼6 to ∼2.5 RS, photodissociation of water from Enceladus and H2 scattered from the ring atmosphere are comparable sources. The newly formed H2+ ions are lost by collisions with the ring particles inside ∼2.5 RS, by interchange processes in the middle magnetosphere, and by flow down the tail in the outer magnetosphere. The density distribution of H2+ estimated from our ion source rates roughly agrees with CAPS observations, and we show that the H2+ density near the equator over the main rings is at least 1 order of magnitude smaller than O2+, possibly consistent with the nondetection of H2+ by CAPS at Saturn orbit insertion.