Coronal holes are regions in the hot solar atmosphere ("corona") where the density of the plasma (i.e. charged particles) at that temperature is very low compared to its surroundings, and thus they look like dark shapes in the corona as imaged in extreme ultraviolet (EUV) or in x-ray. Linked to unipolar, open magnetic fields stretching into space, they are the source of the high-speed solar wind which can create geomagnetic disturbances. One such equatorial coronal hole was in a geo-effective position around 6-8 April (image underneath), and the associated wind stream was the source of a short-lived minor geomagnetic storm on 11 April.
Coronal holes are best observed in EUV wavelengths near 19.3nm, corresponding to temperatures in the corona of more than 1 million degrees. Daily images of the solar corona are made by SDO's AIA instrument, and also with the two STEREO spacecraft as both are equipped with an EUV Imager (EUVI, 19.5nm). STEREO-A and -B image the farside of the Sun, though ST-B is no longer operational after a failure just prior to its superior conjunction (i.e. its passage behind the Sun) back in 2015. See e.g. these STCE news items here and here.
The current position of the three spacecraft can be seen in the sketch above, with SDO orbiting the Earth (the green dot) and A and B representing resp. ST-A and ST-B. Because they make several images every hour, the different points-of-view allow to create an EUV image of the entire Sun every day. These are called "synoptic maps", and are available at the STEREO website. Such a map can be seen underneath, showing the coronal hole from 6-8 April right in the middle of the earth-facing solar hemisphere near 0 degrees longitude and 0 degrees latitude. The black area between longitude 90 and 150 degrees is the area not covered by the combination of the SDO and ST-A images, and would have been visible if ST-B was operational.
Nonetheless, complete synoptic maps were available from February 2011 till September 2014, and reasonably well-covered synoptic maps for pretty much the entire solar cycle 24 (SC24), starting from January 2009 onwards. Hence a movie (underneath) was created showing one synoptic map per solar rotation (27 days) from the start of SC24 till present (April 2018). As can be seen, coronal holes can appear everywhere on the solar surface, contrary to the active (sunspot) regions (bright areas) which are restricted to a band of roughly 30 degrees on each site of the solar equator. One can also observe the much slower rotation at the poles (more than 30 days), with extensions of the coronal holes "trailing" and moving eastward (to the left) compared to the equatorial features.
SC24 started with two well-formed polar coronal holes, as shown in image underneath (top). As the cycle progressed and solar activity started to increase, the polar coronal holes gradually disappeared and small but usually short-lived isolated coronal holes started to emerge between active regions and outside the active region belt. By 2011, the northern polar coronal hole had gone, with the southern polar coronal hole following a year later. The close spacing between and continuous emerging of the active regions during the years of solar cycle maximum limited the widths of the coronal holes and rapidly changed their boundaries (Wang, 2017). See image underneath (bottom) showing only some isolated coronal holes at latitudes below about 45 degrees.
Some researchers have also indicated that polar coronal holes evolve from this interaction between equatorial coronal holes and decaying active region complexes (e.g. Golubeva et al., 2016). Indeed, shortly after solar cycle maximum, polar coronal holes started to gradually develop from some high-latitude (about 60 degrees) coronal holes, first in the southern hemisphere and then in the northern hemisphere. This evolution is not so new, as it has e.g. also been observed during SC23 (see sketch underneath from the Sun's north pole, by Cranmer, 2009, Figure 3). Quite peculiar during SC24 was that the southern polar coronal hole reformed first, though it had dissolved *after* the northern polar coronal hole.
As the solar cycle progressed, some big coronal holes were seen in 2015 and 2016 (see the annual STCE reviews), with the equatorial extensions of the polar coronal holes becoming somewhat smaller and less pronounced during 2017. The polar coronal holes are again well reformed now, but they differ from their predecessors at the beginning of the solar cycle at one crucial point: they are of opposite magnetic polarity, as can be seen in these drawings from NOAA/SWPC (Top: January 2009; Bottom: April 2018), testifying again of the polar magnetic field reversal. Surely, those coronal holes have their say in the solar cycle!
