Effects of ion composition on escape and morphology at Mars

Abstract

Abstract. We refine a recently presented method to estimate ion escape from non-magnetized planets and apply it to Mars. The method combines in-situ observations and a hybrid plasma model (ions as particles, electrons as a fluid). We use measurements from the Mars Atmosphere and Volatile Evolution (MAVEN) mission and Mars Express (MEX) for one orbit on 2015-03-01. Observed upstream solar wind conditions are used as input to the model. We then vary the total ionospheric ion upflux until the solution fits the observed bow shock location. This solution is a self-consistent approximation of the global Mars-solar wind interaction at this moment, for the given upstream conditions. We can then study global properties, such as the heavy ion escape rate. Here we investigate the effects on escape estimates of assumed ionospheric ion composition, solar wind alpha particle concentration and temperature, solar wind velocity aberration, and solar wind electron temperature. We also study the amount of escape in the ion plume and in the tail of the planet. Here we find that estimates of total heavy ion escape are not very sensitive to the composition of the heavy ions, or the amount and temperature of the solar wind alpha particles. We also find that velocity aberration has a minor influence on escape, but that it is sensitive to the solar wind electron temperature. The plume escape is found to contribute 29 % of the total heavy ion escape, in agreement with observations. Heavier ions have a larger fraction of escape in the plume compared to the tail. We also find that the escape estimates scales inversely with the square root of the atomic mass of the escaping ion specie.

The Influence of Solar Irradiation and Solar Wind Conditions on Heavy Ion Escape from Mars

“…(e) and (j) show the ambipolar electric field. Note that the blank area on the nightside region in these three electric field terms has a nonzero resistivity term in our hybrid equation (Q. Zhang et al., 2023).…”

“…To explore the dependence of the escape morphology on the upstream conditions, we here focus on two primary escape channels: tail escape and plume escape, which are defined by a geometric box following the description by Q. Zhang et al. (2023) and shown in Figure 3a. Figure 3b gives an example of the cross section of the tail and plume flux that we use to compute the escape rate of Orb.…”

“…Regions with a plasma density less than 1% of the solar wind density are considered as vacuum regions, with the resistivity of 10 6 Ωm. The ionosphere is composed of upflux of 54% O + , 39% $${\mathrm{O}}_{2}^{+}$$, and 7% $${\mathrm{C}\mathrm{O}}_{2}^{+}$$ (Q. Zhang et al., 2023). The heavy ions are produced on the dayside from the obstacle boundary, with the initial velocity drawn from a Maxwellian distribution corresponding to a temperature of 200 K. The ion upflux decreases by the cosine of the solar zenith angle from the subsolar point to the terminator.…”

“…More model details can be found in Q. Zhang et al. (2023), and more details on the hybrid solver itself can be found in Holmström (2010).…”

“…In this paper, we use a recently proposed method (Holmström, 2022; Q. Zhang et al., 2023) to perform a parameter dependence study. The advantage of this method is that it makes control of the parameters possible by selecting MAVEN orbits based on specified criteria.…”