Changes of the temperature-speed relationship through the inner-heliosphere

Many in-situ solar wind observations show a clear correlation between the proton temperature and the bulk speed. Different authors use various formulas to describe this behavior, but one relation over a speed range covering both the slow and fast solar winds is usually sufficient for measurements made at 1 AU. Moreover, the relationship is observed throughout the solar cycle and no significant variations were measured. Therefore, it is commonly used to predict the expected solar wind temperature for a given solar wind speed. The ratio between the expected and measured proton temperature also serves as one of the ICME identifiers since these often have unusually low temperatures. However, the mechanisms leading to the relationship between the proton temperature and speed are not fully understood. It may be the result of solar wind acceleration processes near the Sun, but it may also change during the solar wind propagation, for example due to the stream interactions and/or different ways of energy transfer in solar wind streams coming from distinct source regions. We use observations from the recent inner-heliosphere missions (Parker Solar Probe and Solar Orbiter) and combine them with the near-Earth measurements (WIND, ACE) to study the radial evolution of this relationship. We find that the character of the radial evolution changes significantly at about 0.4 AU from the Sun. We discuss the possibility that the solar wind reaches a nearly collisionless regime near this point, thus we also investigate the effect of the collisional age.