Neutral H₂ and H₂⁺ ions in the Saturnian magnetosphere

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 H ₂⁺ 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 R_S). From ∼6 to ∼2.5 R_S, 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 R_S, 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 H ₂⁺ 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.