Solar Wind Charge Exchange X-rays at Mars

Wherever the solar wind meets a neutral atmosphere, X-rays are emitted by a charge exchange process between the neutrals and heavy solar wind ions. A hybrid simulation of the solar wind-Mars interaction and a test particle simulation of heavy ion trajectories near Mars is used to compute the contribution from charge exchange processes to the X-ray emission from Mars. The results are compared to observations of X-rays from Mars made with the Chandra telescope (K.~Dennerl, Astronomy & Astrophysics, vol.~394, pp.~1119--1128, 2002). The comparison indicates that the solar wind charge exchange process is a likely candidate for the production of the X-ray halo at Mars. The calculations were performed in three steps. First the solar wind parameters were estimated. We compare the results of two different solar wind parameter estimates: A ballistic model based on data obtained by the WIND spacecraft, and an MHD model using input from interplanetary scintillation measurements. These two models produce X-ray images with significantly different structure. The second step was running a hybrid simulation of the interaction between the solar wind and Mars to obtain the electric and magnetic fields around Mars. As a third step a test particle simulation was run, calculating the trajectories of heavy solar wind ions in the electric and magnetic fields that were obtained from the hybrid simulation. The X-ray emission density was saved on a grid for each time step of the test particle simulation. These simulations show that the contribution from the solar wind charge exchange process to the X-ray emissions from the halo is large enough to explain the observed X-ray flux (H.~Gunell, et al., Geophys. Res. Lett., vol.~31, 2004). Here we study how the simulated X-ray emissions depend on the parameters of the simulation model. The intensity of the X-ray emissions and the size of the X-ray halo are also found to increase with an increasing exobase neutral temperature.