Exospheric neutral density study using XMM-Newton soft x-ray observations and MHD-based magnetosheath model

Abstract

Studying interactions between the solar wind and the Earth is important, and one future satellite mission, called Solar wind Magnetosphere Ionosphere Link Explorer (SMILE), will be launched in late 2024 or early 2025 to study these interactions. SMILE will image the magnetosheath using the charge exchange process and extract the location and motion of the bow shock and magnetopause. The subsolar magnetopause is typically around 10 Earth radii, and neutral density in this outer exosphere is poorly understood. For that reason, we study the neutral density around 10 Earth radii. We estimated the exospheric density at 10 Earth radii using XMM-Newton astrophysics observations, one taken during solar minimum and five taken during solar maximum. For the solar minimum case study, the lower limit of the exospheric density was estimated to be 36.8 ± 11.7 cm⁻³ at 10 Earth radii subsolar point. For solar maximum case studies, we estimated neutral density to be in the range of 42.5 - 65.1 cm⁻³ at 10 Earth radii subsolar point. This suggests weak dependence of neutral density on solar activity but more statistical analysis is needed. The neutral density behavior of the outer exosphere will help us understand the Earth's atmospheric loss due to the dynamic space environment and thus, infer the entire evolutionary history of the Earth's atmosphere as well as of other planetary atmospheres. Along with neutral density, plasma number density, velocity, and temperature in the magnetosheath are key parameters for predicting soft X-ray images. We developed a user-friendly model of magnetosheath parameters to help derivation of these parameters in future wide field-of-view soft X-ray missions. The model parameterizes number density, velocity, magnetic field, and temperature, by using OpenGGCM MHD simulation results as seed data. We made a suite of magnetosheath models, by compiling pre-existing magnetosheath models (analytic, gas-dynamic) with our MHD-based model. This parameterized model is expected to enable researchers to reconstruct expected soft X-ray images and also use these images for analysis of observed images from future satellite missions including SMILE.