Two-dimensional histograms showing the dependence on magnetic field strength, B, of the power of f_alf (top), f_fast (middle), and (bottom). The progressively more red colors indicate a progressively larger number of points in a given interval of values of B and the power of the above quantities, in logarithmic units. Only points with plasma-β below 10 and with magnetic field inclination θ <  40° at heights between 1.1 and 1.3 Mm are selected for the analysis.