EUV observations of the local ISM

This thesis describes the design, calibration, operation and data analysis of two space-borne extreme-ultraviolet photometers, sensitive at the 584 A spectral line of neutral helium. Each photometer employed a helium gas resonant absorption cell whose use allowed the determination of some features of the 584 A line profile being observed. One instrument was launched November 25, 1974, from the White Sands Missile Range. The other was on board the Apollo spacecraft that participated in the Apollo-Soyuz Test Project in July, 1975. Both instruments performed satisfactorily and returned useable data. These data were analyzed by fitting to models which describe the flow of the interstellar medium through the solar system. A new model, valid even when thermal speeds of the interstellar gas atoms are not small compared to the gas bulk speed, was developed for this purpose. Analysis of data from the first instrument indicated that to two standard deviations, the bulk speed of the local interstellar medium with respect to the Sun exceeds 15 km s-1. Data from the second instrument provided a measurement of the number density of neutral helium in the nearby interstellar medium, of 0.0089 +- 0.0049 cm-3, which was almost a factor of three smaller than values derived by cruder measurements. Both of these results are in good agreement with more recent work by other experimenters