Test Particle Analysis of High Altitude Ion Transport and Escape on Mars.

Abstract

Because Mars has a weak magnetic field in comparison with Earth, the solar wind can directly interact with the neutral planetary environment and drive atmospheric erosion. Located in the overlap region of the atmosphere and the solar wind, the neutral constituents of the atmosphere are ionized and instantaneously affected by the fast-moving solar wind and interplanetary magnetic field. These newly-created ions are 'picked up' by the solar wind, and accelerated away from Mars by the solar wind's motional electric field. The main objective of this work is to extensively probe the high altitude ion transport and escape on Mars using a test particle model that tracks the motion and acceleration of pick-up ions through near-Mars space using virtual detectors.

The first focus of this study addresses how the escape of O+ is influenced relative to the production mechanisms: photoionization, charge exchange and electron impact, finding that the total production and loss rates differ up to two orders of magnitude. This dissertation also investigates the influence of the hot oxygen corona and the solar cycle on the individual ion trajectories. This study found that the inclusion of the corona roughly doubles the total escape for solar minimum conditions and directly contributing to high energy sources above 1 keV and increases the O+ flux and total escape by an order of magnitude from solar minimum to maximum. Two other related focal points for this dissertation included examining which species dominates pick-up ion loss from Mars and quantifying how the ionospheric source influences subsequent pick-up ion acceleration. While the results indicate that O+ dominate the loss, the ionospheric species O2+ and CO2+ were most likely to escape.

The simulations have robustly described the physics controlling high altitude ion escape by isolating the influence of ion production, the solar cycle, the ionospheric contributions, the dominant species and the background fields. The results presented are significant for the eventual interpretation of ion observations at Mars in order to quantify how much of the atmosphere is escaping, which is a critical aspect of understanding how water has evolved on Mars.