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

¹ Université Versailles St.-Quentin, Sorbonne Universités, UPMC Univ. Paris 06, CRNS/INSU, LATMOS-IPSL, 11 boulevard d’Alembert, 78280 Guyancourt, France
e-mail: dimitra.koutroumpa@latmos.ipsl.fr
² Space Research Institute of Russian Academy of Sciences, 117997 Moscow, Russia
³ GEPI/Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
⁴ Finnish Meteorological Institute, 00101 Helsinki, Finland

Received: 4 October 2016
Accepted: 15 November 2016

Abstract

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-α 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°± 1.4° throughout the 20-year span of the SWAN dataset, further strengthening the arguments for the stability of the interstellar gas flow.