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 Å). 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 Å (∼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 Å), O II (834 Å), and H I (1216 Å). 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.