Measure of the atmospheric velocity gradient of η Aql. Left: depth for the 17 spectral lines as a function of the pulsation phase, as well as the depth of the cross-correlated profile (black dots). The quantity corresponding to the line depth is slightly different for the Gaussian fit (RV_cc−g) method or the centroid (RV_cc−c), but it provides the same values. Right: amplitude of the radial velocity curves of the 17 spectral lines listed in Table 1, plotted vs. the line depth, at minimum radius. The amplitudes for the RV_cc−g and RV_cc−c curves (template F6) are shown for comparison (red and black dots, respectively) together with the minimum and maximum depth of the cross-correlated profile (horizontal red and black lines). Different fits are processed (see the text for the explanation).