Global and Sequential Imaging Observation of the Earth's Plasmasphere by PHOENIX Onboard EQUULEUS

The Plasmaspheric Helium ion Observation by Enhanced New Imager in eXtreme ultraviolet (PHOENIX) onboard EQUilibriUm Lunar-Earth point 6U Spacecraft (EQUULEUS) performed global imaging observations of the Earth's plasmasphere from a meridian view. PHOENIX is a normal-incidence telescope designed to observe He II emission at 30.4 nm, consisting of a mirror coated with molybdenum and silicon multilayers, a thin metallic filter made of aluminum and carbon, and a microchannel plate detector. This paper provides an overview of the PHOENIX instrument, its in-flight calibration, and initial results of Earth observations. During in-flight calibration, it was found that stray light affected the data when the phase angle between the line of sight and the Sun was small, but a method for its removal was developed using stray light observations. The calibration results confirmed that PHOENIX is optimized for He II observation, with a sensitivity of $1.45\times {10}^{-2}$ cts/s/pix/Rayleigh for He II. It was also demonstrated that PHOENIX is capable of capturing global images of the Earth's plasmasphere with an angular resolution of less than 0.19° and a temporal resolution of less than 1.5 hr. In May 2023, PHOENIX successfully conducted imaging observations of the Earth's plasmasphere while EQUULEUS was on its way to the Earth-Moon Lagrange point 2, revealing the density structure along the dipole-shaped magnetic field lines. Furthermore, the shrinkage of the plasmasphere due to geomagnetic disturbances was captured. This marks the first global imaging of the Earth's plasmasphere using an ultra-small instrument.