Stability of the interstellar hydrogen inflow longitude from 20 years of SOHO/SWAN observations

Aims. A recent debate on the decade-long stability of the interstellar He flow vector, and in particular the flow longitude, has prompted us to check for any variability in the interstellar H flow vector as observed by the SWAN instrument on board SOHO. Methods. We used a simple model-independent method to determine the interstellar H flow longitude, based on the parallax effects induced on the Lyman-alpha intensity measured by SWAN following the satellite motion around the Sun. Results. Our results show that the interstellar H flow vector longitude does not vary significantly from an average value of 252.9 degrees +/- 1.4 degrees throughout the 20-year span of the SWAN dataset, further strengthening the arguments for the stability of the interstellar gas flow.