The Soft X-ray Imager (SXI) on the SMILE Mission

Abstract The Soft X-ray Imager (SXI) is a compact X-ray telescope with a wide $15.5^{\circ } \times 26.5^{\circ }$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mn>15.5</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>26.5</mml:mn> <mml:mo>∘</mml:mo> </mml:msup> </mml:math> field-of-view (FOV) and is part of the scientific payload of the Solar wind Magnetosphere Ionosphere Link Explorer (SMILE) mission. SMILE is a joint science mission between the European Space Agency (ESA) and the Chinese Academy of Sciences (CAS) and is due for launch in 2026. SXI is capable of viewing large portions of Earth’s magnetosphere from the vantage point of the SMILE orbit. SXI is designed to detect the soft X-rays from the Solar Wind Charge eXchange (SWCX) process produced when heavy ions of solar wind origin interact with neutral particles in Earth’s exosphere. SWCX provides a mechanism for boundary detection within the magnetosphere, such as the position of Earth’s magnetopause, because the solar wind heavy ions essentially do not enter regions of closed magnetic field lines. The sensitivity of the SXI is such that it can potentially track movements of the magnetopause on timescales of a few minutes and the orbit of SMILE will enable such movements to be tracked for segments lasting many hours. The SXI has a large grasp (FOV times effective area) and a CCD-based detector system with an energy resolution comparable to contemporary X-ray astrophysics missions. SXI will, therefore, acquire high precision spectra capable of discriminating the composition of many of the solar wind heavy ions within the regions of interest. SXI is led by the University of Leicester (UL) in the United Kingdom (UK) with collaborating organisations on hardware, software and science support within the UK, Europe, China and the United States.