OBSERVATIONS OF THE LOCAL INTERSTELLAR-MEDIUM WITH THE EXTREME-ULTRAVIOLET EXPLORER

Because of the strong absorption of extreme ultraviolet radiation by h ydrogen and helium, almost every observation with the Extreme Ultravio let Explorer (EUVE) satellite is affected by the diffuse clouds of neu tral gas in the local interstellar medium (LISM). This paper reviews s ome of the highlights of the EUVE results on the distribution and phys ical state of the LISM and the implications of these results with resp ect to the interface of the LISM and the heliosphere. The distribution of sources found with the EUVE all-sky surveys shows an enhancement i n absorption toward the galactic center. Individual spectra toward nea rby continuum sources provide evidence of a greater ionization of heli um than hydrogen in the Local Cloud with an mean ratio of H I/He I of 14.7. The spectral distribution of the EUV stellar radiation field has been measured, which provides a lower limit to local H II and He II d ensities, but this radiation field alone cannot explain the local heli um ionization. A combination of EUVE measurements of H I, He I, and He II columns plus the measurement of the local He I density with interp lanetary probes can place constraints on the local values of the H I d ensity outside the heliosphere to lie between 0.15 and 0.34 cm(-3) whi le the H II density ranges between 0.0 and 0.14 cm(-3). The thermal pr essure (P/k = nT) of the Local Cloud is derived to be between 1700 and 2300 cm(-3) K, a factor of 2 to 3 above previous estimates.