Histograms of propagation angle distribution as a function of heliocentric distance and solar wind speed. Panel a: Normalized histogram of the number of whistler waves as a function of the propagation angle for different heliocentric distances. Panel b: Normalized histogram of the number of whistler waves as a function of the propagation angle for different solar wind speeds. Panel c: Same as panel a zoomed between 0 and 20°. Panel d: Same as panel b zoomed between 0 and 20°.