Challenges in identifying the coronal hole wind

Context. Solar wind is frequently categorized based on its respective solar source region. Solar wind that originates in coronal holes is consequently called coronal hole wind. Two well-established categorizations of the coronal hole wind, the scheme based on the charge-state composition, and the scheme based on proton plasma, identify a very different fraction of solar wind in the data from the Advanced Composition Explorer (ACE) as coronal hole wind during the solar activity minimum at the end of solar cycle 24. Aims. We investigate possible explanations for the different identifications of the coronal wind in 2009 in the scheme based on the charge-state composition (almost only coronal hole wind) and in the scheme based on the proton plasma (almost no coronal hole wind at the same time). The high fraction of coronal hole wind observed in the scheme based on the charge-state composition in 2009 was also addressed previously. Methods. We compared the properties of the respective coronal hole wind types and their changes with solar activity cycle in 2001–2010. As a comparison reference, we included the coronal hole wind as identified by an unsupervised machine-learning approach, k -means, in our analysis. Results. Because the ratio of the O 7+ to O 6+ densities drops systematically for the entire solar wind during the solar activity minimum, which cannot be captured by the fixed threshold on the O charge-state ratio suggested in the scheme based on the charge-state composition, we find that this solar wind classification likely misidentifies some slow solar wind as coronal hole wind during the solar activity minimum. The k -means coronal hole wind agrees with the very low fraction of coronal hole wind observed in the classification based on the proton plasma. In addition, the k -means classification we considered includes two types of coronal hole wind, the first of which is dominant during the solar activity maximum and exhibits comparatively higher O and Fe charge states, whereas the second is dominant during the solar activity minimum and features lower O and Fe charge states. A low fraction of coronal hole wind from low-latitude coronal holes observed by ACE in 2009 is plausible because during this time period, a very small number of low-latitude coronal holes was observed. The scheme based on proton plasma and 7-means also relies on fixed decision boundaries, wherein the decision boundary in the scheme based on the proton plasma for coronal hole wind appears to be better adapted for conditions at solar activity minimum than maximum. Conclusions. The results imply that the origin-oriented solar wind classification needs to be revisited, and they also suggest that an explicit inclusion of the phase of the solar activity cycle can be expected to improve the classification of the solar wind.