Coronal Holes, Footpoint Reconnection, and the Origin of the Slow (and Fast) Solar Wind

Abstract

The tendency for low-speed solar wind to show greater spatiotemporal variability and different compositional properties from high-speed wind has led to the prevailing idea of a bimodal solar wind, in which fast wind comes from coronal holes and slow wind comes from coronal streamers. We present observational evidence that most of the slow wind originates from small coronal holes or from just inside the boundaries of large holes, with the rest leaking out from coronal streamers and confined to the immediate vicinity of the heliospheric current and plasma sheets. Although this conclusion was suggested earlier by extrapolations of photospheric field maps, additional support comes from (1) observations of slow wind at Earth following the central-meridian passage of small equatorial holes; (2) observations of slow wind with high Alfvénicity at 1 au by Wind, and more recently near the Sun by Parker Solar Probe and Solar Orbiter; and (3) the finding that 80% of the solar wind observed by Helios at 0.3 – 0.4 au during 1974 – 1978 was Alfvénic. We show that compositional properties such as charge-state ratios vary over the solar cycle and may depend on parameters such as the footpoint field strength $B_{0}$ B 0 , and thus cannot be used alone to distinguish between coronal hole and noncoronal-hole wind. Finally, we note that magnetograms greatly underestimate the amount of small-scale flux emerging inside coronal holes and other unipolar regions. If this rate is taken to be the same as in the quiet Sun, the energy flux density resulting from interchange reconnection with open field lines is on the order of $3\times 10^{5}$ 3 × 10 5 erg cm−2 s−1 ($B_{0}$ B 0 /10 G), sufficient to drive the solar wind. The wind speed depends on the rate of flux-tube expansion, with slower expansion leading to more energy deposition at greater heights and faster wind.