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Proton-proton collisional age to order solar wind types

The properties of a solar wind stream are determined by its source region and by transport effects. Independently of the solar wind type, the solar wind measured in situ is always affected by both. We consider the proton-proton collisional age as an ordering parameter for the solar wind at 1AU and explore its relation to the solar wind classification scheme developed by Xu & Borovsky (2015). We use this to show that explicit magnetic field information is not required for this solar wind classification. Based on the observation that the three basic solar wind types from this categorization cover different regimes in terms of proton-proton collisional age $a_{col,p-p}$ , we propose a simplified solar wind classification scheme that is only based on the proton-proton collisional age. The resulting so-called PAC solar wind classifier is an alternative to the full Xu & Borovsky (2015) solar wind classification scheme and leads to a classification that is very similar to the full Xu & Borovsky (2015) scheme. The solar wind is well ordered by the proton-proton collisional age. This implies underlying intrinsic relationships between the plasma properties, in particular, proton temperature and magnetic field strength in each plasma regime. We argue that sector-reversal plasma is a combination of particularly slow and dense solar wind and most stream interaction boundaries. Most solar wind parameters (e.g., the magnetic field strength, B, and the oxygen charge state ratio $n_{O^{7+}}$ /$n_{O^{6+}}$) change with the solar activity cycle. Thus, all solar wind categorization schemes based on threshold values need to be adapted to the solar activity cycle as well.