PHEBUS: Probing of Hermean Exosphere By Ultraviolet Spectroscopy

Computer Science – Performance

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Abstract The double spectrometer PHEBUS covers the range of Extreme Ultraviolet (55-155 nm) and Far Ultraviolet (145-315 nm) [1]. Focusing on the characterisation, composition, dynamics and surface-exosphere coupling of Mercury, PHEBUS addresses the following main scientific objectives: determination of the composition and the vertical structure of the exosphere, characterization of the exospheric dynamics: day to night circulation, active to inactive regional transport, study of surface release processes, identification and characterization of the sources of exospheric constituents, detection and characterization of the ionosphere and its relation with the neutral atmosphere, space and time monitoring of exosphere/magnetosphere exchange and transport processes, study and quantification of escape, global scale source/sink balance and geochemical cycles synergistically with other experiments of BepiColombo (MSASI, MPPE on MMO and MIXS, SERENA on MPO). Instrumentation PHEBUS consists of two gratings and two detectors which are fitted within a very compact design. The spectrum detection is based on the photon counting method using Micro Channel Plate (MCP) detectors with Resistive Anode Encoder (RAE) with photocathode's coated with CsI for the EUV range and CsTe for the FUV range. Extra visible lines (K and Ca) are monitored using a Photomultiplier (PM) that is also used in photon counting mode. In order to prevent sensitivity losses which are critical in UV ranges, a minimum of reflections is achieved inside the instrument using only an off-axis parabola and a set of holographic gratings. A one degree-offreedom scanning system allows probing at the highest possible signal to noise ratio in the selected regions and altitude ranges of interest. Different modes of observation will be used sequentially (vertical scans, along-orbit scans, grazing observations at twilight…). During the mission, the instrument will be regularly calibrated on well chosen stars to quantitatively estimate the overall degradation of the sensitivity of the instrument. Science Performance To evaluate the science performance of the PHEBUS a radiometric model has been developed. The results from that model will be presented. References [1] Chassefière, E. et al. (2008) Planetary and Space Science,

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