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

TL;DR

This study analyzes 20 years of SWAN data to assess the stability of the interstellar hydrogen flow longitude, finding no significant variation and supporting the notion of a stable interstellar gas flow vector.

Contribution

It provides a model-independent analysis confirming the long-term stability of the interstellar hydrogen flow longitude using SWAN observations.

Findings

Interstellar H flow longitude remains stable over 20 years.

No significant variation in flow longitude detected.

Supports the hypothesis of a stable interstellar gas flow.

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-{\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$^{\circ}$ $\pm$ 1.4$^{\circ}$ throughout the 20-year span of the SWAN dataset, further strengthening the arguments for the stability of the interstellar gas flow.