Development of a compact EUV photometer for imaging the planetary magnetosphere

We have developed a lightweight, compact, and highly sensitive Extreme ultr aviolet (EUV) photometer for a space-borne instrument. The photometer is on board Japan's Mars orbiter, Planet-B, which was successfully launched on Ju ly 4, 1998, and stayed in the parking orbits around the Earth for 6 months. The new photometer is designed for studying the spatial distribution of he lium ions and atoms by detecting He Il (304 Angstrom) and He I (584 Angstro m), which are major emission lines detectable around a planet. The photomet er is a type of normal-incidence telescope that consists of a mirror, filte rs, and a detector. The mirror employs a new technology of a molybdenum/sil icon multilayer coating and is designed to have peak reflectivity at 304 An gstrom. The photometer is capable of taking two-dimensional images by using the spin and orbital motions of the spacecraft. There are open questions c oncerning the Earth's plasmasphere and plasma sheet that cannot be resolved by in situ observations alone (e.g., global shape of the plasmasphere and cold ions in the plasma sheet). Our observations with the EUV photometer ov er a reasonable number of orbits will answer them. The photometer also show s an outstanding performance of measuring the Martian helium.