Interplanetary Lyman alpha line profiles derived from SWAN SOHO hydrogen cell measurements: Full-sky Velocity Field

We present here an analysis of 1 year of data obtained by the solar wind an isotropies (SWAN) instrument on board the SOHO spacecraft orbiting around t he LI Lagrange point at 1.5 x 10(6) km sunward from Earth. This instrument is measuring the interplanetary Lyman alpha background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interpla netary (IP) Lyman alpha line profile reflects the velocity distribution of H atoms projected onto the line of sight (LOS). Here we apply a new profile reconstruction technique using data from the two hydrogen absorption cells included in the SWAN instrument. For a LOS in a fixed celestial direction, the Doppler shift between the interplanetary emission profile and the H ce ll absorption profile varies by up to +/-0.12 Angstrom during 1 year, owing to the Earth's orbital velocity around the Sun, equal to 30 km s(-1). Such a Doppler spectral scan across the emission line allows us to derive Lyman alpha line profiles, and hence the velocity distribution, in and out of th e ecliptic independent of any modeling of the neutral hydrogen atom distrib ution in the heliosphere or of the multiple scattering of solar photons. Th e spatial distribution of,the apparent velocity relative to the Sun as obse rved from the orbit of SOHO, is derived for all directions, except within 5 degrees of the ecliptic poles. This determination strongly constrains mode ls of the interaction of the interstellar hydrogen with the solar wind. New estimates of the upwind direction (252.3 degrees +/- 0.73 degrees and 8.7 degrees +/- 0.90 degrees in J2000 ecliptic coordinates) show a small discre pancy by 3 degrees - 4 degrees with the direction of the helium flow, perha ps connected with an asymmetry of the heliosphere induced by the interstell ar magnetic field. We find that the apparent velocity relative to the sun i n the upwind direction is -25.4 +/- 1 km/s, whereas it is equal to 21.6 +/- 1.3 km s(-1) in the downwind direction. A preliminary analysis shows that the Zero Doppler shift cone and the difference between the upwind and downw ind velocities correspond to a ratio mu of Lyman alpha radiation pressure t o solar gravity of 0.9-1.0. It follows that the observed upwind apparent ve locity is compatible with a velocity at infinity of H atoms of the order of 21-22 km s(-1). However, extensive modeling is required in order to get mo re definite conclusions. The velocity map presented here is the first ever obtained. For this reason, we discuss in detail the Doppler spectral scan m ethod and the H cell data.