Interplanetary Lyman α line profiles derived from SWAN/SOHO hydrogen cell measurements: Full‐sky Velocity Field

Abstract

We present here an analysis of 1 year of data obtained by the solar wind anisotropies (SWAN) instrument on board the SOHO spacecraft orbiting around the Ll Lagrange point at 1.5 × 106 km sunward from Earth. This instrument is measuring the interplanetary Lyman α background due to solar photons backscattered by hydrogen atoms in the interplanetary medium. The interplanetary (IP) Lyman a 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 cell absorption profile varies by up to ±0.12 Å 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 α line profiles, and hence the velocity distribution, in and out of the ecliptic independent of any modeling of the neutral hydrogen atom distribution in the heliosphere or of the multiple scattering of solar photons. The spatial distribution of the apparent velocity relative to the Sun as observed from the orbit of SOHO is derived for all directions, except within 5° of the ecliptic poles. This determination strongly constrains models of the interaction of the interstellar hydrogen with the solar wind. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° in J2000 ecliptic coordinates) show a small discrepancy by 3° – 4° with the direction of the helium flow, perhaps connected with an asymmetry of the heliosphere induced by the interstellar magnetic field. We find that the apparent velocity relative to the sun in 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 downwind velocities correspond to a ratio μ of Lyman α radiation pressure to solar gravity of 0.9–1.0. It follows that the observed upwind apparent velocity 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 more definite conclusions. The velocity map presented here is the first ever obtained. For this reason, we discuss in detail the Doppler spectral scan method and the H cell data.