H i Lyα radiance profile along the UVCS slit from the exposure containing the nucleus taken near perihelion (red squares and blue diamonds represent data taken north and south of the cometary nucleus). The green dashed curve is a model (Model A) obtained by considering two hydrogen atom populations (v_H1 = 20 km s^-1, v_H2 = 8 km s^-1) with Q_H2O and τ_H left as free parameters. The green solid curve is a model (Model B) obtained by adding the contribution of a thermalized hydrogen atom population (v_H3 = 4 km s^-1). See the text for details.