SWAN/SOHO H cell measurements: The first year Original Research Article

We present here an analysis of one year of data obtained by the SWAN instrument on board the SOHO spacecraft orbiting around the L1 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 α line profile reflects the velocity distribution of H atoms, projected on 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. New estimates of the upwind direction (252.3° ± 0.73° and 8.7° ± 0.90° J2000 ecliptic) show a small discrepancy by 3° to 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 in the downwind direction.