Observation of He II emission from the plasmasphere by a newly developed EUV telescope on board sounding rocket S‐520‐19

The helium emission monitor (HEM) is a newly developed extreme ultraviolet telescope adjusted to be sensitive to the He II emission line (304 A). It is developed as a prototype for upcoming plasmaspheric imagery missions. The HEM is designed to be compact and simple using a multilayer coated mirror, a thin metal filter, and a microchannnel plate coated with CsI; it exhibits high sensitivity at 304 A (∼100 cps/R) and high angular resolution (2.5°), and most importantly it is contamination free from other major emission lines, e.g., He I (584 A), O II (834 A), and H I (1216 A). We report on the initial observation of He II emission from the plasmasphere by the HEM carried on board 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 observations using values for the He+ number density at the topside ionosphere of 3700 cm−3 and for the equatorial temperature near the dawnside of the plasmapause of 8000 K.