Advances in soft X-ray emission of the Earth's magnetosphere

The coupling between solar wind and the magnetosphere and the dynamic processes in geo-space are the basic driving factors of space weather. Understanding these processes on the system level is essential to the studies of space physics and space weather. Recent solar wind charge exchange (SWCX) X-ray emission discoveries provide a novel approach to detect the large-scale magnetosphere through soft X-ray imaging. SWCX occurs when high-charge heavy ions such as C6+, N7+, O7+, and O8+ in the solar wind interact with neutral atoms or molecules, such as neutral hydrogen atoms in near-Earth space, neutral hydrogen and helium atoms in the heliosphere, and H2O and CO2 molecules on comets and other planets. Solar wind ions become excited by receiving one or more electrons, and then return to the ground state by releasing one or more photons in the soft X-ray band. The characteristics of the SWCX emissions include the X-ray spectrum showing line emissions corresponding to different species of solar wind particles and neutrons, and fast time variations closely related to solar wind variations. The SWCX soft X-ray emission of the Earth's magnetosphere mainly occurs in the magnetosheath on the dayside and the cusp regions. Therefore, the magnetosphere can be remotely imaged using large-scale soft X-ray imaging technology, allowing the fundamental modes of the interaction between the solar wind and magnetosphere to be recognized on a systematic scale.In this context, the European Space Agency (ESA) and Chinese Academy of Sciences (CAS) jointly proposed the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE). SMILE was approved in 2016 and implemented thereafter. SMILE aims to provide remote sensing measurements of the magnetopause and bow shock around the subsolar region, part of the cusp regions, and the aurora, as well as simultaneous in situ observations of the solar wind plasma and magnetic field. SMILE is planned to be launched in 2024–2025. Other missions have also been proposed to image the Earth's magnetosphere, such as the GEOspace X-ray imager (GEO-X) in Japan, Lunar Environment heliospherics X-ray Imager (LEXI) in the United States, and Lunar-based soft X-ray imager (LSXI) in China.With the advent of soft X-ray imaging missions to detect the magnetopause, evaluating the expected soft X-ray images and developing appropriate techniques to extract 3-dimentional boundary information from the 2-dimentional images is essential. Global magnetohydrodynamic (MHD) and instrument simulations are used to generate expected X-ray images under different solar wind conditions with different viewing geometries. Based on these images, several approaches have been developed to analyze the signals, composing the 'arsenal' or 'toolkit' for magnetopause reconstruction. Each approach has its own advantages and disadvantages, applicable to different situations.This paper introduces the mechanism of magnetospheric X-ray emission, reviews the research on the observational evidence and characteristics of SWCX X-ray emissions, summarizes the progress of simulation studies, presents the missions (or concept) of the magnetospheric X-ray detection, and discusses the possibility of future implementation of X-ray imaging for detecting other planets.