Coronal Holes and Jets

Abstract

The term coronal hole has at least three definitions: (i) the darkest patches on the solar surface as measured in UV and soft x-rays; (ii) the lowest intensity regions measured above the limb, seen either during a total solar eclipse or with an occulting coronagraph; and (iii) open-field (or open-flux) regions, which are equivalent to the footpoints of the time-steady solar wind flows (for reviews see Cranmer 2009; Wang 2009). The term “coronal holes” (or “koronale Locher” in German) was coined by Max Waldmeier around 1956, based on the reduced brightness of the white-light corona in polar regions as seen during (total) solar eclipses. The area on the disk covered by the open magnetic field is largest during the minimum of the solar cycle, when the global field becomes mostly poloidal and covers the (northern and southern) polar caps. However, coronal holes occur not only in the polar regions, but occasionally extend to lower latitudes and may even bridge the equator (trans-equatorial holes). Besides the large-scale coronal holes, there is also some evidence of open magnetic flux in the vicinity of active regions (Fisk and Zurbuchen 2006; Wang 2017).