Exospheric heating by pickup ions at Titan

Abstract

Titan has a very extensive atmosphere and exosphere which interact strongly with the corotating magnetospheric plasma. Some of the new pickup ions created in the vicinity of the exosphere will re-impact the upper atmosphere causing additional energy input. Previous investigations of the atmospheric collisional interaction of the pickup ions have generally assumed the atmosphere to be spherically symmetric. From the Voyager and Cassini observations, we know that the magnetic field configuration and plasma flow field is highly asymmetric. To study the possible spatial variation in the pickup ion influx, we have employed the plasma data of the three dimensional MHD simulation of Kopp and Ip [Kopp, A., Ip, W.-H. Asymmetric mass loading effect at Titan’s ionosphere. J. Geophys. Res. 106, 8323–8332, 2001] to compute the trajectories of the pickup ions. In this work, we calculate the ion influx and energy deposit into Titan’s exobase for the $\text{H}_2{}^{+}$, $\text{CH}_4{}^{+}$ and $\text{N}_2{}^{+}$ pickup ions separately. The model results of four different Titan’s orbital locations are also presented.

Introduction

As a planetary satellite, Titan is unique in the sense that it has a very substantial atmosphere. The surface pressure is ∼1500 mbars and its chemical composition is mainly nitrogen with a small fraction of CH₄ (∼2.7%) and other minor species (Waite et al., 2005). Because of its lack of an intrinsic magnetic field, Titan has direct atmospheric interaction with the Saturnian magnetosphere or the solar wind (Ness et al., 1982, Neubauer et al., 1984). Since the first in-situ measurements made by the Voyager 1 spacecraft, much effort has been spent on the detailed numerical investigations of the physical processes of Titan’s plasma–atmosphere interaction (Luhmann, 1996, Ledvina and Cravens, 1998, Ledvina and Cravens, 2005, Kabin et al., 1999, Kabin et al., 2000, Kopp and Ip, 2001, Nagy et al., 2001, Ma et al., 2006). The issue of atmospheric sputtering is of particular interest. This is because the ejection of neutral atoms and molecules from the exobase will lead to the generation of an extended corona and the formation of a gas torus in circumplanetary space (Barbosa, 1987, Lammer and Bauer, 1993, Sittler et al., 2004, Smith et al., 2004, Michael et al., 2005). At the same time, the collisional interaction of the incoming energetic ions with Titan’s upper atmosphere could lead to a significant level of heating. Some of these dynamical effects have been recently investigated by Ledvina and Cravens, 2005, Michael and Johnson, 2005, Michael et al., 2005, Sillanpää et al., 2006. For example, Michael and Johnson (2005) used a Direct Simulation Monte Carlo (DSMC) method to study the energy deposit and heating of the upper atmosphere of Titan. They showed that the pickup ions can deposit more energy near the exobase than solar radiation. However, due to the loss of the energy by escaping neutrals and collisional transport to the lower atmosphere, the temperature increase near the exobase is only a few degrees. Such sputtering heating might be related to the enhancement of neutral density as observed by the Cassini INMS experiment (Waite et al., 2005). Ledvina and Cravens (2005) used the numerical output from a 3D single-fluid MHD model (Ledvina and Cravens, 1998) combined with a test particle/Monte Carlo model to study the distribution of the pickup ions in the vicinity of Titan. These authors found that Titan’s exobase absorbs 1.4 × 10²² 1 amu ions per second, 5.6 × 10²³ 14 amu ions per second, and 8.7 × 10²³ 28 amu ions per second, respectively.

The many close encounters of the Cassini spacecraft with Titan will provide a wealth of information on the interaction of Titan’s atmosphere with Saturn’s magnetosphere. Hartle et al. (2006) analysed the data from the CAPS instrument obtained during the TA flyby. They found that many of the basic plasma features were similar to the results from the Voyager 1 encounter (Sittler et al., 2005). In particular, the atmospheric absorption pattern of the ambient O⁺ ions displayed a certain asymmetry which might be caused by the finite gyroradius effect. It is important to note that under Titan’s plasma conditions with an ambient magnetic field strength of 5 nT and a plasma flow speed of about 120 km s⁻¹, the gyroradius of a new N⁺ ion is comparable to the size of Titan. This has important consequences. As a result of the orientation of the convective electric field the heavy pickup ions newly created in the Saturn-facing hemisphere will likely re-impact Titan’s atmosphere. But those new pickup ions created in the opposite hemisphere will be carried away from Titan by the corotating plasma flow. As mentioned before, the exosphere could be subject to an additional heating effect because of the re-impacts of the pickup ions. The finite gyroradius effect would then produce an asymmetric energy deposit on Titan’s exobase. In this work, we will report on the results obtained from a series of computer simulations with a view to study the asymmetric energy deposits from pickup ions under different plasma flow conditions. In Section 2, the 3D MHD model and the test particle code will be briefly described. In Section 3, the results for the situation of a uniform source distribution of pickup ions will be compared to those for Titan interactions at different orbital positions around Saturn. Finally, a general discussion and a summary will be given in Sections 4 Discussions, 5 Summary.