Estimating the Subsolar Magnetopause Position from Soft X-ray Images using Lowpass Image Filter

The Lunar Environment Heliospheric X-ray Imager (LEXI, lunar flatform) and Solar wind-Magnetosphere-Ionosphere Link Explorer (SMILE, high apogee Earth-orbiter) will take photos of the Earth’s dayside magnetopause and cusps in soft X-rays after their respective launch in 2024 and 2025 for understanding global magnetic reconnection modes under varying solar wind conditions. To support successful science closure, it is critical to develop techniques to extract magnetopause position from observed soft X-ray images. In this presentation, we introduce a new method that derives subsolar magnetopause position (SMP) as a function of a satellite location and a look direction that gives peak soft X-ray emission. Two assumptions are used in this method: 1. The look direction of maximum soft X-ray emission is the tangent to magnetopause, 2. the magnetopause near a subsolar point is nearly spherical and thus the SMP is equal to the radius of magnetopause curvature. We test this magnetopause tracing method by using the anticipated LEXI soft X-ray images under various solar wind conditions. First, we simulate synthetic soft X-ray images observed from various LEXI locations using the OpenGGCM global magnetosphere MHD model. Galactic background, particle background, and Poisson noises are considered in these images. Then, we apply a lowpass filter to the synthetic LEXI images for removing noises and obtaining accurate look angles of soft X-ray peaks. From filtered images, we calculate SMPs for various LEXI locations and solar wind fluxes, and estimate its accuracy by using the SMPs of OpenGGCM as ground truth. Our method estimates SMPs with an accuracy of <0.3RE and this accuracy improves as the solar wind density increases.