Observation and occurrence of whistlers as a function of heliocentric distance and solar wind speed. Panel a: Number of analyzed 8s-waveforms with Solar Orbiter as a function of heliocentric distance and solar wind speed. Panel b: Whistler wave occurrence as a function of heliocentric distance for sunward, anti-sunward, and poorly defined propagation direction. When, due to low statistics, the 95% confidence intervals (calculated assuming a normal distribution) are of the order of magnitude of values, they are not shown. Panel c: Occurrence as a function of the solar wind speed.