Determining the Subsolar Magnetopause Position Using CMEM

Abstract

The wide field-of-view soft X-ray imager on the upcoming SMILE mission will revolutionize our understanding of magnetopause dynamics from observations of SWCX in the magnetosheath. Inferring the position of the 3D magnetopause boundary in the 2D images is challenging, but several methods have been developed to do this. One method generates images through a 3D emissivity model and adapts its parameters to achieve the best fit with real images. The subsolar magnetopause position is extracted from the fitted model. We show that the Wharton et al. (2025, https://doi.org/10.1029/2024ja033307), cusp and magnetosheath emissivity model can be used successfully for this purpose in a wide range of scenarios. The method works when varying the image resolution, the orientation of the imager around its pointing axis, the aim point of the imager, and for a range of solar wind densities. We also show the method works from a range of orbital positions with realistically constrained viewing geometries. Finally, we tested the method on ensembles of realistic X-ray counts images which contain noise and instrumental contributions, allowing us to determine the uncertainty in the subsolar magnetopause distance over a wide range of solar wind densities. We found that CMEM accurately determined the subsolar magnetopause distance whenever it lies within the field-of-view and the signal-to-noise ratio was sufficient to observe the magnetosheath. The standard deviation of the subsolar magnetopause distance was less than 0.25 $R_E$ whenever the subsolar magnetopause was visible.