Exploration of variability in the line shape and Doppler shift of scattered solar Lyman α radiation from interplanetary Hydrogen

We present an updated analysis of high-resolution observations of Lyman α emission of interplanetary hydrogen (IPH). The data was obtained in March 2001, using HST/STIS in the E140H mode with a large aperture (52”x0.5”). This unsupported mode blends spectral features from different orders of the echelle grating but greatly increases the signal-to-noise ratio in the resulting spectrum. The neutral IPH is composed primarily of material originating from the local interstellar medium (LISM). The relative motion between the solar system and the local cloud in which it is embedded results in a neutral interstellar wind, that penetrates the solar system from the direction λ=254°, β=7° (in ecliptic coordinates). The interstellar wind crosses the heliosheath the boundary that separates the solar wind and the magnetized plasma component of the LISM, and then flows into the solar system. Several decades of study have shown that the velocity distribution of the neutral IPH is substantially altered by charge exchange with interstellar ions during the transit across the heliosheath, and then by gravity and radiation pressure as this flow approaches the Sun. The most precise observations reveal deceleration of the IPH relative to interstellar hydrogen. HST/GHRS observations (1994 and 1995) show an extra component, interpreted as the spectral signature of a Fermi process occurring in the heliospheric interface (Ben Jaffel et al. 2000). A recent comparison of line resolved data obtained by the SOHO/SWAN instrument and HST/STIS points to a time dependent change in the average velocity (Quemerais et al. 2006). Several analyses of Voyager data obtained between 1978 and mid-2007 have also shown intensity variations of Lyman-alpha emission with solar activity cycle. In this work, we compare the line resolved spectrum of backscattered solar Lyα radiation observed with HST/STIS in 2001 near solar maximum, with similar data obtained with HST/GHRS in 1994 and 1995 near solar minimum. We present a detailed examination of the line shape, searching for a Fermi emission similar to that seen in the GHRS observations and a change in the average velocity or velocity distribution. We find that in the long slit STIS observations, both the geocoronal and IPH HI lines are contaminated by overlapping light from geocoronal OI emission in lower orders, affecting any structure we can see. A second echelle order containing Lyman-alpha emission appears in the spectrum, and with only the geocoronal HI line affected. This provides partial line profiles that can be used to study the shape. This contamination by orders overlapping must be taken into account when searching for small features as the Fermi emission or the geocoronal Deuterium emission.