Challenges in identifying the coronal hole wind

Abstract

Solar wind is frequently categorized based on its respective solar source region. 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. 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). 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. We find that the scheme based on charge-state composition likely misidentifies some slow solar wind as coronal hole wind during the solar activity minimum. The k-means classification we considered includes two types of coronal hole wind, the first of which is dominant during the solar activity maximum, whereas the second is dominant during the solar activity minimum. 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 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.