Ultraviolet Imager (UVI) for the SMILE Mission

The Ultraviolet Imager (UVI) is one of the four instruments onboard the Solar wind Magnetosphere
Ionosphere Link Explorer (SMILE) satellite, a collaborative science mission between
the Chinese Academy of Sciences (CAS) and the European Space Agency (ESA).
The UVI will capture the terrestrial auroral images that depict the energy depositions in
the solar wind-magnetosphere coupling system. The primary function of UVI is to image
the entire auroral oval at characteristic wavelengths while effectively mitigating contamination
from dayglow, achieving a spatial resolution of approximately 100 km or better. The
co-axis four-mirror all-reflective optical system provides a circular field of view of 9.97°,
enabling coverage of the entire polar region when the spacecraft’s geocentric distance exceeds
50,000 km. This capability allows UVI to continuously monitor the complete auroral
oval for over 40 hours. UVI operates in the long wavelength range of the N2 Lyman-Birge-
Hopfield (LBH) band, specifically between 160–180 nm. To achieve the required spectral
response and significantly reject out-of-band stray light, multilayer coatings are applied to
its mirrors. The detector utilized is an intensified charge-coupled device (ICCD), with photons
emitted from the phosphor being coupled through a relay lens to the ICCD. By default,
the CCD captures one frame (512 × 512 pixels) every two seconds; thus, thirty frames per
minute are coadded to produce a single UVI image. This processing can be performed in
orbit or on ground to compensate the possible satellite jitters. The detailed geometric and
photometric calibration procedures for UVI are elaborated upon in the paper.