Contour plots of the quality factor Q (in logarithmic scale) of periodically driven Alfvén waves in the upper solar chromosphere as function of the frequency f (vertical axis in logarithmic scale) and height above the photosphere (horizontal axis). Top and botton panels correspond to the left and right polarized waves, respectively. Left and right panels are the results obtained using the hard-sphere cross sections (HS) and the Vranjes & Krstic (2013) cross sections (VK), respectively. The black dashed line corresponds to the approximate optimal frequency given in Eq. (43). We note that the VK results are only strictly valid for heights larger than 900 km.