Si IV doublet averaged over visits B_fuv and C_fuv, as a function of velocity relative to the line centers (vertical dotted lines). The upper panel shows the Si IV λ1394 line. The lower panel shows the Si IV λ1403 line. Pre-transit spectra (blue) show a flux decrease compared to the in- and post-transit spectra (black), consistent with absorption from an optically thin cloud of ionized silicon gas. The green (before convolution) and red (after convolution) profiles correspond to a best-fit model for the intrinsic stellar line (solid line) and the occulting cloud (dashed line).