Small Coronal Holes Near Active Regions as Sources of Slow Solar Wind

• Published in: The Astrophysical journal Vol. 841; no. 2; pp. 94 - 111
• Main Author: Wang, Y.-M.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-06-2017; IOP Publishing
• Subjects: Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Boundaries; Coronal holes; EXTRAPOLATION; EXTREME ULTRAVIOLET RADIATION; Fluctuations; HELIOSPHERE; LIMBS; MAGNETIC FIELDS; MAGNETIC FLUX; Photoionization; Photosphere; Radiation; SOLAR CORONA; Solar limb; SOLAR WIND; STREAMS; SUN; Sun: activity; Sun: corona; Sun: heliosphere; Sun: magnetic fields; Sun: UV radiation; Sunspot activity; SUNSPOTS; VELOCITY; Wind speed
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa706e

We discuss the nature of the small areas of rapidly diverging, open magnetic flux that form in the strong unipolar fields at the peripheries of active regions (ARs), according to coronal extrapolations of photospheric field measurements. Because such regions usually have dark counterparts in extreme-ultraviolet (EUV) images, we refer to them as coronal holes, even when they appear as narrow lanes or contain sunspots. Revisiting previously identified "AR sources" of slow solar wind from 1998 and 1999, we find that they are all associated with EUV coronal holes; the absence of well-defined He i 1083.0 nm counterparts to some of these holes is attributed to the large flux of photoionizing radiation from neighboring AR loops. Examining a number of AR-associated EUV holes during the 2014 activity maximum, we confirm that they are characterized by wind speeds of ∼300-450 km s−1, O7+/O6+ ratios of ∼0.05-0.4, and footpoint field strengths typically of order 30 G. The close spacing between ARs at sunspot maximum limits the widths of unipolar regions and their embedded holes, while the continual emergence of new flux leads to rapid changes in the hole boundaries. Because of the highly nonradial nature of AR fields, the smaller EUV holes are often masked by the overlying canopy of loops, and may be more visible toward one solar limb than at central meridian. As sunspot activity declines, the AR remnants merge to form much larger, weaker, and longer-lived unipolar regions, which harbor the "classical" coronal holes that produce recurrent high-speed streams.