Effects of Heliolatitudinal Anisotropy of Solar Far-ultraviolet/Extreme-ultraviolet Emissions on Lyα Helioglow

Abstract

We present a study of the influence of solar UV anisotropy on the heliospheric backscatter helioglow generated by resonant scattering of solar Lyα photons on interstellar hydrogen atoms around the Sun. Simulations based on the WawHelioGlow model suggest that the response of the helioglow pole-to-ecliptic ratio to the anisotropy is linear, but 15% of the anisotropy (polar darkening) generates 30%–40% change in the ratio in the solar minimum and 15%–20% in the solar maximum. We attribute this difference to an interplay between the solar UV anisotropy and the latitudinal structure of the solar wind in solar minima. The solar UV anisotropy also increases the helioglow intensity from the downwind direction by 5%–10%, due to the influence of the anisotropy on the ionization losses and trajectories of atoms passing by the Sun in polar regions. Consequently, midlatitude regions (in the heliographic and ecliptic coordinates) are least affected by the UV anisotropy. By comparison of the simulation results with observations of the Solar and Heliospheric Observatory/SWAN satellite instrument, we derive the day-by-day time evolution of the solar Lyα anisotropy for the north and south poles over two solar cycles from 1996 to 2022. The inferred anisotropy is ∼5%–10% in solar minima and ∼15%–25% in solar maxima, the northern anisotropy being stronger than the southern one. Our study suggests that in solar minima a highly structured solar wind is associated with relatively small solar UV anisotropy, while in solar maxima the solar wind is more isotropic but a substantial solar UV anisotropy appears.