Influence of Heliolatitudinal Anisotropy of Solar FUV/EUV Emissions on Lyα Helioglow: SOHO/SWAN Observations and WawHelioGlow Modeling

Abstract Observations of the Sun’s surface suggest a nonuniform radiated flux as related to the presence of bright active regions and darker coronal holes. The variations of the FUV/EUV source radiation can be expected to affect the Lyα backscatter glow measured by spaceborne instruments. In particular, inferring the heliolatitudinal structure of the solar wind from helioglow variations in the sky can be quite challenging if the heliolatitudinal structure of the solar FUV/EUV radiation is not properly included in the modeling of the heliospheric glow. We present results of analysis of the heliolatitudinal structure of the solar Lyα radiation as inferred from comparison of SOHO/SWAN satellite observations of the helioglow intensity with modeling results obtained from the recently developed WawHelioGlow model. We find that in addition to time-dependent heliolatitudinal anisotropy of the solar wind, time-dependent heliolatitudinal variations of the intensity of the solar Lyα and photoionizing emissions also must be taken into account to reproduce the observed helioglow modulation in the sky. We present a particular latitudinal and temporal dependence of the solar Lyα flux obtained as a result of our analysis. We also analyze differences between polar-equatorial anisotropies close to the solar surface and seen by an observer located far from the Sun. We discuss the implications of these findings for the interpretation of heliospheric-glow observations.