Abstract
The solar wind originating from coronal holes is comparatively well-understood and is characterized by lower densities and average charge states compared to the so-called slow solar wind. Except for wave perturbations, the average properties of the coronal-hole solar wind are passably constant. In this case study, we {ocus on observations of the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE) of individual streams of coronal-hole solar wind that illustrate that although the O and C charge states are low in coronal-hole wind, the Fe charge distribution is more variable. In particular, \markmeold{we illustrate that the Fe charge states in coronal-hole solar wind are frequently as high as in slow solar wind. We selected individual coronal-hole solar wind} streams based on their collisional age as well as their respective O and C charge states and analyzed their Fe charge-state distributions. Additionally, with a combination of simple ballistic back-mapping and the potential field source surface model, transitions between streams with high and low Fe charge states were mapped back to the photosphere. The relative frequency of high and low Fe charge-state streams is compared for the years 2004 and 2006. We found several otherwise typical coronal-hole streams that include Fe charge states either as high as or lower than in slow solar wind. Eight such transitions in 2006 were mapped back to equatorial coronal holes that were either isolated or connected to the northern coronal-hole. Attempts to identify coronal structures associated with the transitions were so far inconclusive.
Highlights
The steady solar wind is typically divided into two dominant types, fast and slow solar wind
To prevent any interference by other processes, we focus on the part of the coronal-hole-wind stream from day of year (DoY) 4.5−6.5 where both density ratios are safely below their respective categorization thresholds
Complementing the observations of the long-term behavior of the Fe charge states in von Steiger et al (2000), Galvin et al (2009), Richardson (2014), Lepri et al (2013), Kasper et al (2012), Schwadron et al (2011), Zhao & Landi (2014), we present a case-study of individual solar-wind streams that can be clearly identified as coronal-hole wind, with either high or low Fe charge states compared to the charge states in slow solar wind
Summary
The steady solar wind is typically divided into two dominant types, fast and slow solar wind. A simple model to explain the observed solar wind speeds and charge states assumes that the charge state can change along the solar-wind flux tube until the expansion time scale (which depends on the electron density profile in the corona) is of the same order as the charge modification time scale of an ion pair. Beyond this point the charge-state distribution remains “frozenin” as the solar wind propagates further outwards. We map these streams back to their coronal sources and assess whether there are any coronal structures that may be associated with these transitions
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