Proton-proton collisional age as a tracer for solar wind type

The properties of an individual solar wind stream depend on the conditions of the respective source region of that stream and on transport effects. Historically, at least two types of solar wind are considered: coronal hole wind (originating from polar or equatorial coronal holes) and slow solar wind. For the latter case, the source regions and mechanisms have not been uniquely identified. Assuming only two types of solar wind is simplistic and has been challenged frequently (D'Amicis & Bruno 2015, Sanchez-Diaz et al. 2016, Heidrich-Meisner et al. 2018). Established solar wind categorization schemes fall into two categories: 1) Composition-based approaches rely on the charge state composition (Zhao et al. 2009). 2) (Proton) plasma properties can be exploited to determine solar wind type (Xu & Borvosky 2014, Camporeale et al. 2017). Notably, many solar wind categorization schemes tend to disregard transport effects, e.g. stream interaction regions.

Here, we explore the relation of the proton-proton collisional age (which has been proposed as a suitable ordering parameter for solar wind in Kasper et al 2008, 2017) to the Xu & Borovsky 2014 solar wind categorization scheme. The Xu & Borovsky scheme discerns four types of solar wind: ejecta plasma (which mainly represents interplanetary coronal mass ejections), coronal hole wind, and two types of slow solar wind: sector-reversal plasma (plasma that contains a current sheet crossing) and streamer belt plasma. We focus here on the three background solar wind types (coronal hole wind, sector reversal and streamer belt) and show that all three can be separated by the proton-proton collisional age of the solar wind. This indicates a relationship between magnetic field strength and proton temperature for all three solar wind types that is caused by the different plasma conditions in each type.