Data analysis and model fitting. Top: brightness for the entire Lyα slit along the y-axis, averaged over the x-axis (i.e., over the slit width). Middle: Zoom-in of the top panel. Bottom: brightness along the x-axis, averaged over a y range covering the disk diameter. The STIS data are shown as a black histogram. The fitted model is represented by dashed lines corresponding to different components: green for the background only, blue for the background plus H exosphere contribution, and red for the full model including surface reflection and IPH correction (see text). Note: the profiles are smoothed due to averaging over a detector axis (and, thus, across the disk), as well as by the convolution of the model image with the STIS point spread function. The vertical dotted lines indicate the disk center in all panels. The model was fitted within the central disk and the gray shaded regions (see top panel).