Coronal holes (CH) are regions in the hot solar atmosphere ("corona") where the plasma density at that temperature is very low compared to its surroundings, and thus they look like dark shapes in the corona. Linked to unipolar magnetic fields stretching into space, they are the source of the high-speed solar wind and can create geomagnetic disturbances.
The image underneath is an SDO image (AIA 193) in extreme ultraviolet (EUV) of the solar corona and the coronal hole on 18 July. It is overlaid with white lines that are approximations for the magnetic field lines as deduced from magnetic fields from the solar surface. Clearly, one can see how in the (bright) active regions, the magnetic field lines are bound to the solar surface ("closed" loops), whereas over the coronal hole, the field lines are extending into space ("open").
One of the larger coronal holes so far this solar cycle was visible during the summer months of this year (see e.g. this STCE Newsitem of 25 July). In fact, it has made 6 transits across the solar disk. Underneath SDO-images (AIA 193) from 23 May, 20 June, 18 July, 14 August, 10 September and 7 October showing the coronal hole near the Sun's central meridian.
With the STEREO spacecraft observing the backside of the Sun, forecasters have a permanent view on the evolution of active regions and large scale structures such as coronal holes. This movie covers the period from 1 May till 18 October. It shows a projection of STEREO and SDO EUV imagery of the solar surface onto a flat map. All features transit from east to west (left to right), with the part between -90 and +90 degrees being directed to the Earth. One can see the development and subsequent evolution of the coronal hole. It was largest during its second passage (June), during which it had almost 360 times the surface area of the Earth! It is only now, 4 months later, that STEREO-images indicate this CH has almost completely vanished.
A coronal hole’s high-speed solar wind stream can produce severe geomagnetic disturbances. However, in this case, the effects of the 700km/s stream were mostly limited to active conditions. Only during its first and fourth transit, a (brief) minor geomagnetic storm was recorded, whereas during its last passage, the CH does not seem to have had any influence at all. It must be noted that Earth was experiencing the effects of a coronal mass ejection at that time (8-9 October), which may have masked some of the CH’s influence (if any).
Interestingly, during its first four passages, the high-speed stream brought a whole bunch of high-energy electrons with it (see this STCE Newsletter for more details). Satellite operators noted a significant increase in anomalies during these periods, thought to be due to repeated electrostatic discharges. Fortunately, none of the spacecraft was permanently damaged.
Credits - Data and imagery were taken from SDO, LMSAL, STEREO, Helioviewer, SWPC, and SIDC.
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