EUVE observations of the helium glow: Interstellar and solar parameters

We present new observations of the diffuse He I 58.4 nm background recorded in 1998 and 2000 by the Extreme Ultraviolet Explorer (EUVE). This emission is due to resonant scattering of the solar EUV radiation by interstellar and geocoronal helium. Depending on the geometry and relative velocity, a fraction of the interstellar helium glow can be absorbed by the line-of-sight geocoronal gas. The new results are combined with measurements obtained in 1992-93 and previously analyzed by Flynn et al. ( 1998). A kinetic model of the helium flow is now used to analyze the data and reproduce the absorption features due to geocoronal helium. This allows a precise determination of the interstellar flow bulk velocity vector. A model that includes both photoionization and electron impact ionization was fit to the data set. New constraints on the interstellar helium flow temperature and density, as well as on the solar 58.4 nm line width are obtained. The interstellar helium velocity vector parameters, lambda = 74.7 +/- 0.5degrees, beta = -5.7 +/- 0.5degrees, V-He = 24.5 +/- 2 kms(-1), are found to be in good agreement with those derived from particle measurements. Using the solar He I 58.4 nm flux and photoionization rate proxies of McMullin et al. (2003), the neutral helium density and temperature derived from the Long Wavelength Spectrometer data is n(He) = 0.013 +/- 0.003 cm(-3), and T-He = 6500 +/- 2000degrees respectively, again in good agreement with particle data. However, the width of the downwind cone when scanned across the latitudnal direction tends to be fit better with higher He temperatures, which might indicated latitude anisotropies in the He ionization that we have not included in our models. The solar He I 58.4 nm Doppler width, Deltaw(D), is found to be = 0.0074 nm, (or 38 +/- 3 kms(-1)) in 1992-1993, i.e. near solar maximum, and Deltaw(D) = 0.0087 nm (45 +/- 3 kms(-1)) in 1998, after solar minimum, in agreement with SOHO SUMER and CDS results, although again, the 1998 fits near solar minimum might suffer from latitudinal anisotropies.