Time evolution of the average of the difference in the mean temperature between the ambipolar and MHD simulations (left panel) and the increase in temperature in a unit of time corresponding to the ambipolar heating h_a (right panel). The averages are done in the z-direction between the heights 0.8 and 1.7 Mm, and in the x-direction. To ease the comparison, the quantities have been normalized using the value r, defined in Eq. (19).