Effect of the HI Lyα chromospheric flux anisotropy on the total intensity of the resonantly scattered coronal radiation

In modeling of the resonantly scattered solar coronal Ly alpha line of H I, the intensity of the chromospheric source is often assumed to be uniform. We investigate the validity of this assumption. After establishing a correlation between the H I 121.6 nm and He II 30.4 nm line intensities, we build Carrington maps of the Ly alpha chromosphere from SOHO EUV Imaging Telescope data. These maps are used to compute the Ly alpha irradiance throughout the corona and heliosphere. A 15% latitudinal anisotropy is found at 1 AU at solar minimum, and this value becomes larger closer to the Sun. The effect of the flux anisotropy on the total intensity of the Ly alpha resonantly scattered coronal radiation is quantified. We find that at solar minimum, the uniform-disk assumption leads to systematic overestimates of the total intensity of the polar regions by 15% on average. The evolution of this effect with solar activity and the case of other resonantly scattered coronal lines are discussed.