1. How does heavy ion composition influence escape estimates?
The heavy ion composition of the upper parts of an ionosphere directly influences the composition of escaping plasma, as well as the dynamics of the escaping plasma due to their different mass per charge ratios. Carlsson (2006) found that O + is the most abundant escaping species. Measurements show uncertainties in the composition of escaping ions, making it important to explore the influence of different heavy ion compositions on escape estimates. In our model, we specify the ratio between different species, in addition to the total upflux, to fit the bow shock location. We consider O +, O + 2, and CO + 2 since those are the major observed ion species. The total exobase ion upflux and composition used in simulations can be found in Table A. As the O + upflux fraction increases, the total escape rate increases by 45%, suggesting that escape scales inversely with the square root of the atomic mass of the escaping ion specie. An unrealistic run with only CO + 2 exobase upflux resulted in an escape estimate that is 58% smaller than the O + case, supporting the scaling hypothesis. The escape rates of different species are not perfectly scaled due to differences in trajectories and mass per charge. The total energy flux of escaping ions may be similar, independent of the species, indicating that the same power is transferred from the upstream solar wind to the escaping ions. The escape estimate is not highly sensitive to the exact O + /O + 2 ratio of the exobase upflux, so we use 54% O +, 39% O + 2, and 7% CO + 2 for the composition of the exobase upflux in our model.
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2. How does solar wind velocity aberration affect Mars-solar wind interactions?
Solar wind velocity aberration introduces an asymmetry in the ionosphere model, affecting heavy ion escape and global solar wind interactions. The aberration angle can vary, with a median value of 4.7 degrees and up to 15 degrees. MAVEN data from 4117 orbits (November 2014-November 2019) show a velocity aberration distribution. Comparing different cases, the absence of aberration (Case 4) increases escape by 7% compared to Case 6. Rotating the upstream IMF to maintain the same cone angle (Case 5) results in a 1% difference in escape. The primary effect of different aberrations is the angle between the upstream solar wind velocity and the magnetic field, altering the upstream convective electric field. The tilt of the magnetosphere due to velocity aberration is more significant further behind the planet, affecting the global interaction. If the angle between the upstream magnetic field and velocity changes, it impacts the interaction, likely due to the altered upstream convective electric field.
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3. How does the ambipolar field affect ion loss in Mars' collisionless ionosphere?
The ambipolar field plays a significant role in the solar wind interaction with Mars by enhancing ion loss in the collisionless ionosphere above the exobase. Ergun (2016) demonstrated that increased O + 2 outflow occurs with higher high-altitude electron temperature. Xu (2021) utilized electrostatic potential measurements from MAVEN to estimate the global ambipolar field at Mars, which aligns well with MHD model predictions (Ma, 2019). However, the ambipolar field cannot be self-consistently represented in MHD and hybrid models due to the assumptions of charge neutrality. Different approaches are used to approximate the effects of the ambipolar electric field in hybrid models. In our model, the electron pressure is isotropic and computed from the charge density, leading to an electric field that is largest in regions with the largest charge density gradient. This electric field can accelerate ions to escape energies, with accelerations of nearly 0.6, 0.3, and 0.2 km/s 2, as observed by Kar (1996). The ambipolar field in our model is directed outward, but some observations suggest an inward direction in the IMB region due to the electron pressure gradient from the colder ionosphere to the hotter magnetosheath (Xu, 2021). The electron temperature, and therefore the electron pressure, is a free parameter in hybrid models. Future studies could explore alternatives to the adiabatic approximation, such as assuming an electron temperature profile in the ionosphere (Bosswetter, 2004; Modolo, 2016) or solving a fluid flow equation for the electrons (Brecht, 2017). Sensitivity tests with different upstream electron temperatures show that the escape rate increases with higher electron temperatures, indicating the importance of model assumptions regarding the ambipolar fields in escape estimates.
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4. What is the morphology of escaping ions at Mars?
The morphology of escaping ions at Mars consists of two major outflow channels: a cold fluid-like outflow in the tail behind the planet and a more energetic outflow in the direction of solar wind convective electric field. The ion plume at Mars, accelerated by the convective electric field, has been observed by MAVEN and modeled by multifluid MHD and hybrid codes. The separation of tail and plume fluxes is defined by energy, with plume escape contributing 30% to total escape in low EUV conditions and 20% in high EUV conditions. The radial escape, defined as the outflow perpendicular to the X-axis, does not depend on the solar cycle but occurs around 20% to 40% of the total escape. The plume escape rate is 1.96x10^24 s^-1, accounting for 29% of the total escape, with O+ being dominant in total and tail escape, and O+2 being dominant in plume escape.
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