Capella (d = 13pc) was observed in the 170 Å to 450 Å band by a sounding rocket borne spectrometer with a spectral resolution of 35 Å FWHM. Minimum flux sensitivity varies with wavelength and a 2σ upper limit is 0.25 and 0.6 photons cm-2s-1 at 180 Å and 304 Å respectively. The prominent stellar emission line in this bandpass is He II 304 Å. The Capella flux at earth in this line is expected to be in the range 2.8 to 5.4 photons cm-2s-1 in the absence of interstellar extinction. Interstellar absorption was computed from the Copernicus satellite measure of the neutral hydrogen column density, N(H I) = 1.2 x 1018cm-2, towards Capella and a ratio of N(He I)/N(H I) = 0.1. The 50% attenuation at 304 Å results in a flux at earth of 1.4 to 2.7 phcm-2s-1.
No significant Capella flux was detected in the bandpass despite inflight verification of instrument's sensitivity obtained from observations of the He II 304 Å geocoronal emission. The most reasonable explanation for the absence of the 304 Å line is that absorption in the intervening medium exceeds that predicted from N(He I) = 0.1N(H I). A lower limit to the Capella line of sight N(He I) is determined from the 304 Å observation. We find that N(He I)/N(H I) is greater than 0.4. A helium to hydrogen abundance ratio of 0.1 is retained if the hydrogen ionization in the local interstellar medium (LISM) exceeds 60%. Several models of the LISM which incorporate this ionization fraction are treated for self consistency, as well as compatibility with observations of the temperature and ionization fraction of the interplanetary medium. The most plausible model consists of a local, warm (10,000 K), gas embedding the solar system, surrounded by a hot (105 → 106 K) plasma. Ionization of hydrogen in the warm cloud must be maintained by a local interstellar radiation field.
