Ejection of nitrogen from Titan's atmosphere by magnetospheric ions and pick-up ions
Introduction
The flow of plasma onto the atmosphere of a planet or planetary satellite produces a series of energy transfer events that can lead to atmospheric loss, a process often called atmospheric sputtering (e.g., Johnson, 1990, Johnson, 1994). When an energetic ion intersects Titan's exobase and collides with an atmospheric atom or molecule, a direct transfer of momentum occurs which initiates a cascade of collisions between atmospheric particles. During this process atoms or molecules at the exobase can be knocked upward into ballistic trajectories populating the corona or can have sufficient momentum in the right direction in order to escape the gravitational field. Atmospheric sputtering can be produced by impacting solar wind ions, pick-up ions or magnetospheric plasma ions, and produces, for instance, the Io torus and Na cloud (McGrath et al., 2004). Using Voyager 1 observational data Hartle et al. (1982) found evidence for mass loading of the Saturn's magnetospheric plasma by material from Titan's upper atmosphere. Later Neubauer et al. (1984) showed that the incident plasma interacts with the atmosphere of Titan. The interaction of magnetospheric ions and pick-up ions with the atmosphere of Titan, studied here, results in the escape of nitrogen atoms and molecules (Shematovich et al., 2003). These neutrals form a toroidal cloud of nitrogen that is a distributed source of heavy ions for Saturn's magnetosphere Barbosa, 1987, Lammer and Bauer, 1993, Sittler et al., 2004, Smith et al., 2004.
Calculations of the atmospheric loss induced by the co-rotating magnetospheric ions, with assumed energies of 2.9 keV, showed that the atmospheric sputtering rate was much smaller than the photo-dissociation-induced escape rate (Shematovich et al., 2001). However, taking in account the slowing of the heavy, co-rotating plasma ions close to Titan (energies less than 750 eV) and the re-impact of heavy, newly created atmospheric pick-up ions (energies less than 1.25 keV), Shematovich et al. (2003) used a 1-D model to show that atmospheric sputtering could be as important as or dominate the photo-dissociation-induced loss. This is critical as molecular nitrogen ejection only occurs efficiently by atmospheric sputtering. Therefore, the character of the nitrogen plasma trapped in Saturn's magnetosphere will differ depending on the relative importance of these atmospheric loss processes. A 3-D Monte Carlo model is developed here to describe the sputtering of Titan's atmosphere by the deflected magnetospheric N+ and N+2 pick-up ions. The flux of escaping particles is typically formed over a wide transition region in which the character of the gas flow changes from a thermospheric collision dominated regime to an exospheric collision-less regime. In this paper we calculate the production of suprathermal atoms and molecules, their escape and their supply to the nitrogen torus. The implications of these results for Saturn's neutral clouds and magnetospheric plasma are discussed.
Section snippets
Monte Carlo model
A three-dimensional Monte Carlo model is developed to simulate the penetration of ions into Titan's atmosphere. The cascade of collisions initiated by the incoming ions and the recoil atoms and molecules are described. In this simulation only suprathermal nitrogen with a kinetic energy above 0.1 eV is followed in order to limit the computing time. These particles move under Titan's gravity and collide with N2 in the atmosphere. The incident and recoil particles are followed from 1700 to about
Results and discussion
It was initially thought that the co-rotating N+ ions () would be the most efficient sputtering agents at Titan. However, the slowed and deflected N+ and the newly created pick-up ions interact more efficiently with atmospheric molecules and the area from which ions have access to the atmosphere is larger. Hartle et al. (1982) provided observational evidence for pickup ions of mass 29 having velocity components towards Titan, where the direction of electric field points towards Titan.
Summary
We calculated the ejection of N and N2 due to the bombardment of Titan's atmosphere by slowed and deflected magnetospheric N+ and by the molecular pick-up ions (C2H+5, HCNH+ or N+2). The atmospheric recoils are set in motion by momentum transfer collisions and by collisional dissociation. Earlier we showed that the plasma-induced sputtering of Titan is an important contribution to the atmospheric loss rate. Using the 3-D model and a simplified description of the flow, it is observed the total
Acknowledgments
This work is supported by NASA's Planetary Atmospheres Program and by a travel grant from the NSF International program. This is also partially supported by RFBR project 02-02-16087. The author F.L. is supported by ISSI/Switzerland.
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