PHEBUS observation of Helium in Mercury’s Exosphere during the first fly-by of Mercury by the Bepi-Colombo mission

On October 1st 2021, Bepi-Colombo performed its first fly-by of Mercury. During the maneuver, the short wavelength channel of PHEBUS (55nm-155nm) was activated for a total duration of 1 hour. The boresight of PHEBUS was in a fixed direction with respect to the stars during the whole sequence, pointing close to the north ecliptic pole and almost tangential to the limb at the equator. The helium resonance line at 58.4 nm was clearly observed during the whole sequence. At large distance from the planet, the emission was due to helium atoms in the interplanetary medium (interplanetary UV glow). Just after crossing the terminator of the planet and entering the dawn side of the exosphere, PHEBUS observed a clear additional emission due to scattering of solar photons by helium atoms in the exosphere of Mercury. This signal was visible for about 10 minutes, decreasing as Bepi-Colombo moved away from the planet. This dataset contains the total count rate recorded by PHEBUS during the whole observation sequence, as well as the associated dark count rate and the uncertainty for the total count rate, both at 58.4 nm. It also contains some auxiliary data (Mercury and Bepi-Colombo positions, Phebus boresight and Sun direction) useful for the reconstruction of the geometry of the observation.