I. Yoshikawa, M. Nakamura, M. Hirahara, Y. Takizawa, K. Yamashita, H. Kunieda, T. Yamazaki, K. Misaki, A. Yamaguchi

Abstract

Helium emission monitor (HEM) is a newly developed extreme ultra-violet telescope adjusted to be sensitive to He II emission line (304Å). It is developed as a proto-type for upcoming plasmaspheric imagery missions. The HEM is designed to be compact and simple using a multi-layer coated mirror, a thin metal filter, and an MCP coated with CsI, exhibits high sensitivity at 304Å , and most importantly it is contamination free from other major emission lines, e.g., He I (584Å), O II (834Å), and H I (1216Å). We report on the initial observation of He II emission from the plasmasphere by the HEM carried onboard sounding rocket S-520-19. We compare the observation with the theoretical prediction using a diffusive equilibrium model, the MSIS-86 model, and an equatorial electron density model. The model indicates a good agreement with the observation using values of He number density at the topside ionosphere as 3700cm and equatorial temperature near the dawnside of the plasmapause as 8000K.