Relation between the solar wind speed (assumed constant beyond 1 au), and the angular distance to the HCS at 0.1 au δ. The blue normalized histogram map comes from Ulysses latitudinal scan of 2007 while the orange comes from OMNI 1-hr averaged data for the year 2018–2019–2020. The plain black and dashed black curves represent the logistic function described in equation (6), with values of d = 10° and d = 35°, which seem to cover the most significant part of the transition between slow and fast wind streams.