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

TL;DR

This study analyzes how the nonuniform, heliolatitudinal variations in solar FUV/EUV emissions influence the Lyman-alpha helioglow, using SOHO/SWAN observations and the WawHelioGlow model to improve heliospheric diagnostics.

Contribution

It introduces a comprehensive modeling approach that accounts for time-dependent heliolatitudinal variations of solar Lyman-alpha emissions, enhancing interpretation of helioglow observations.

Findings

Heliolatitudinal anisotropy significantly affects helioglow modulation.

Time-dependent variations of solar Lyman-alpha emissions are crucial for accurate modeling.

Differences between polar and equatorial anisotropies impact helioglow interpretation.

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 Lyman-alpha 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 Lyman-alpha 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, also time-dependent heliolatitudinal variations of the intensity of the solar Lyman-alpha and photoionizing emissions 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 Lyman-alpha flux obtained as a result of our analysis. We analyze also 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.