Solar wind categorization schemes rely on different solar wind properties to identify the solar wind type and adopt mainly one of the following two approaches: (1) Composition-based schemes exploit the oxygen and carbon charge-state composition of the solar wind (Zhao & Fisk 2010; von Steiger et al. 2000). Based on the assumption that the charge states observed in the solar wind are determined in the solar corona and are not significantly changed thereafter, lower (higher) charge states are associated with source regions of the respective solar wind stream with comparatively low (high) electron temperatures. Because the charge state is not expected to change during the travel time of the solar wind, composition-based criteria are well suited to identify different solar source regions. Transport effects due to, for instance, compression regions in stream interaction regions and wave-particle interaction are not directly reflected in the charge-state composition. Stream interaction regions tend to be characterized by a (gradual or abrupt) transition in the oxygen charge-state compositions. From the charge-state information alone, stream interaction regions cannot be uniquely identified, and their solar source regions cannot be unambiguously determined without additional or context information. (2) Proton plasma properties provide an alternative to determine the solar wind type (Xu & Borovsky 2015; Camporeale et al. 2017). A clear advantage of this approach is that the required observables are available from more spacecraft. Unlike the charge-state composition, the proton speed, proton density, proton temperature, and magnetic field strength (and derived quantities such as the specific entropy and the Alfvén speed) are all susceptible to transport effects. In particular, these quantities show radial gradients throughout the heliosphere (Marsch et al. 1982; Bale et al. 2019; Kasper et al. 2019). Thus, a solar wind categorization based on threshold values for these quantities can be expected to depend on position. In particular, the solar wind proton temperature is not a tracer of the coronal (electron) temperature. The solar wind proton temperatures show the opposite effect (von Steiger et al. 2000): high solar wind proton temperatures are observed for coronal hole wind (which originates from comparatively cool coronal regions), while low solar wind proton temperatures appear in the slow solar wind (which likely originates in hot coronal regions). The solar wind proton temperature is probably strongly influenced by transport effects, in particular, by wave-particle interactions. In addition, proton temperature, proton density, and magnetic field strength all show characteristic variations in stream interaction regions. Solar wind categorization schemes based on proton plasma properties are therefore well suited to assess the effect of solar wind evolution during its travel time. However, these transport effects can blur the tracers of the solar origin of the solar wind. For approaches based on charge-state composition and on proton plasma, the respective threshold values are usually determined heuristically and vary in the literature. Mainly as a result of their availability, solar wind electron data (e.g., Lin et al. 1995; Wilson et al. 2018) are typically not considered for solar wind classifications, although their properties, for example, the electron temperature and the electron-proton collisional age, can be expected to be informative in this context. Future improvements on solar wind classification would most likely benefit considerably from including electron data. The collisional age (or Coulomb number) has been proposed as an ordering parameter for the solar wind in Kasper et al. (2008), Tracy et al. (2016), and Maruca et al. (2013). The collisional age can be interpreted as counting the number of 90°-equivalent collisions during the travel time from the Sun to the observing spacecraft. This notion of the collisional age relies on the simplifying assumption that the solar wind parameters are constant during the solar wind travel time. Maruca et al. (2013) introduced an improvement in the computation of the collisional age that takes into account that the underlying quantities are not constant during the travel time of the solar wind.
Citation: Xu & Borovsky 2015
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