Example of the observations used in this work. Top right: continuum intensity I_c in the Fe I line at 617.3 nm normalized to the average value in the quiet Sun I_c, qs in the same line. Top left: core intensity I_core in the Ca II line at 854.2 nm, normalized to the average value in the quiet Sun I_c, qs. Bottom left: line-of-sight velocity obtained from the core of the Fe I line at 617.3 nm. Bottom right: line-of-sight velocity obtained from the core of the Ca II line at 854.2 nm. Points indicated with the + and ▫ symbols are discussed in Sect. 4.4.