Time series of μ(v) profiles of C IV computed from the spectra of images A and B obtained at the same epoch. Four epochs are emphasized: May 2003 (blue), May 2006 (cyan), October 2008 (magenta), and February 2018 (red); the other μ(v) profiles computed from spectra obtained in 2003 and 2004 are shown in green and black, respectively. The half error bars of μ(v) are drawn from zero for clarity.