On the apparent line-of-sight alignment of the peak X-ray intensity of the magnetosheath and the tangent to the magnetopause, as viewed by SMILE-SXI

The Soft X-ray Imager (SXI) on board the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) spacecraft will be able to view the Earth's magnetosheath in soft X-rays. Simulated images of the X-ray emission visible from the position of SMILE are created for a range of solar wind densities by using 3 years of the SMILE mission orbit, together with models of the expected X-ray emissivity from the Earth's magnetosheath. Results from global magnetohydrodynamic simulations and a simple model for exospheric neutral densities are used to compare the locations of the lines of sight along which integrated soft X-ray intensities peak with the lines of sight lying tangent to surfaces (defined here to be the magnetopause) along which local soft X-ray intensities peak or exhibit their strongest gradients, or both, for strongly southward interplanetary magnetic field conditions when no depletion or low-latitude boundary layers are expected. Where, in the parameter space of the various times and seasons, orbital phases, solar wind conditions, and magnetopause models, the alignment of the X-ray emission peak with the magnetopause tangent is good, or is not, is presented. The main results are as follows. The spacecraft needs to be positioned well outside the magnetopause; low-altitude times near perigee are not good. In addition, there are seasonal aspects: dayside-apogee orbits are generally very good because the spacecraft travels out sunward at high altitude, but nightside-apogee orbits, behind the Earth, are bad because the spacecraft only rarely leaves the magnetopause. Dusk-apogee and dawn-apogee orbits are intermediate. Dayside-apogee orbits worsen slightly over the first three mission years, whereas nightside-apogee orbits improve slightly. Additionally, many more times of good agreement with the peak-to-tangent hypothesis occur when the solar wind is in a high-density state, as opposed to a low-density state. In a high-density state, the magnetopause is compressed, and the spacecraft is more often a good distance outside the magnetopause.