Modeling of the O⁺pickup ion sputtering efficiency dependence on solar wind conditions for the Martian atmosphere

Abstract

Sputtering of the Martian atmosphere by O+pickup ions has been proposed as a potentially important process in the early evolution of the Martian atmosphere. In preparation for the Mars Atmosphere and Volatile Evolution (MAVEN) mission, we performed a study using a Monte Carlo model coupled to a molecular dynamic calculation to investigate the cascade sputtering effects in the region of the Martian exobase. Pickup ion fluxes based on test particle simulations in an MHD model for three different solar wind conditions are used to examine the local and global sputtering efficiencies. The resultant sputtering escape rate is 2×1024 s−1 at nominal solar wind condition and can be enhanced about 50 times when both the interplanetary magnetic field (IMF) strength and the solar wind pressure increase. It is found that when the IMF strength becomes stronger, both the pickup ion precipitation energies and the resultant sputtering efficiencies increase. The related escape flux, hot component, and atmospheric energy deposition deduced from the MAVEN measurements may reveal clues about the prominent enhanced sputtering effects. Significant hemispheric asymmetries can be observed related to the solar wind electric fields. The shielding by the crustal fields and the recycling onto the nightside due to different magnetic field draping features can also lead to regional variations of sputtering efficiencies. The results suggest that disturbed or enhanced solar wind conditions provide the best prospects for detecting sputtering effects for MAVEN mission.

Statistical studies on Mars atmospheric sputtering by precipitating pickup O+: Preparation for the MAVEN mission

“…Although different descriptions of the initial atmospheres may influence the solar wind interactions and the pickup ion precipitation distributions, their impact on the sputtering processes is limited when the same incident ion precipitates on different atmospheres as long as the major component near the exobase region is CO 2 [ Wang et al , ]. The 1‐D atmospheric sputtering model is developed by Leblanc and Johnson [] and is extended to 3‐D by Wang et al []. Detailed explanations on the calculations of the collisions including the impact parameters and interaction potential can be found in Johnson and Liu [], Johnson et al [], Leblanc and Johnson [], and Wang et al [].…”

“…Combining a hybrid model, hot corona calculations, and sputtering simulations, Chaufray et al [] also deduced that the sputtered hot corona and escape rates of O are currently smaller than those from the dominant dissociative recombination of under normal solar wind interactions. Recent studies that included enhanced solar wind, such as the passing of interplanetary coronal mass ejections (ICMEs), solar energetic particle event, or corotating interaction regions (CIRs), suggested that the sputtering effects may significantly increase for short periods due to vary large transient ion precipitation fluxes and energies [ Leblanc et al , ; Hara et al , ; Wang et al , ]. On the other hand, the existence of the surface crustal fields and their various interactions with the IMF can affect the pickup ion escape rates and the precipitation energy and fluxes as well as the resulting sputtering efficiencies [ Ma et al , ; Ma and Nagy , ; Fang et al , ; Li et al , ].…”

“…To derive the sputtering effects from the reimpact of the pickup O + on the neutral upper atmosphere, models for the solar wind interaction, pickup ion transport, and the neutral atmosphere collisional cascade calculations are needed. Since each of these individual model has been described and their results published, detailed information can be found in previous papers [ Ma et al , ; Leblanc and Johnson , ; Fang et al , ; Wang et al , ]. We thus provide only a short introduction to each of the model and their specific features in section 2.…”

Influence of Martian crustal magnetic anomalies on the emission of energetic neutral hydrogen atoms

“…However, these enhancements are expected to be negligible since the local cusp regions are much smaller in comparison to the total area of the magnetic anomalies [Brain et al, 2007], and instances of large scale heights are rare according to radio occultation observations [Ness et al, 2000]. Enhancements of the neutral density due to sputtering are also expected to be negligible for nominal solar conditions at Mars [Johnson et al, 2008;Wang et al, 2014]. Another source of the exospheric density enhancement is via the modification of chemical reactions in the magnetic anomaly regions with closed field lines.…”

O+ ion beams reflected below the Martian bow shock: MAVEN observations

“…Due to their large gyroradii, they escape Mars by forming a plume-like structure in the direction of the convection electric field [Fang et al, 2008;Dong et al, 2015] [Fang et al, 2010;Leblanc et al, 2015]. It is known that the precipitating O + ions are a source of ion sputtering in the upper atmosphere of Mars [Luhmann and Kozyra, 1991;Wang et al, 2014]. Hara et al [2011] showed that O + pickup ions precipitate toward Mars.…”