A Monte Carlo model of energy deposition, ionization, and sputtering due to thermal ion precipitation into Titan's upper atmosphere

Abstract

A three-dimensional model of ion precipitation is developed to calculate incoming energy flux, number flux, charge exchange rates, energy deposition rates, and sputtering rates due to thermal (<10 keV) ion precipitation into Titan's upper atmosphere. We report results for a range of upstream conditions that represent Titan's environment in Saturn's plasma sheet. Our results show that Titan's induced magnetosphere is effective at shielding the atmosphere from thermal proton precipitation. However, the atmosphere is only partially shielded from thermal oxygen precipitation. The energy deposited by thermal magnetospheric oxygen (between about 100 to 400 MW) is smaller than solar EUV input but similar to the energy deposited by high energy neutral atoms and magnetospheric electrons. Oxygen deposits the most energy on the ram-side of Titan's atmosphere where we calculate an average energy deposition rate within 45° longitude of co-rotational ram-point between 17 and 85 eV cm−3 s−1. The number flux of oxygen atoms is ~1 × 105 cm−2 s−1, normalized to Titan's surface area. Maximum local charge exchange rates are around 0.5 cm−3 s−1, which is smaller than photoionization and impact ionization rates from magnetospheric electrons. We also test the model's sensitivity to various assumptions about cross-sections, collision parameters, and the characteristics of Titan's atmosphere. Our results imply that the energy deposition and charge exchange rates are variable due to fluctuations in Titan's atmospheric structure and its environment in Saturn's magnetosphere. The most significant source of uncertainty in the Monte Carlo collision algorithm is the collision scattering angle.