Coronal Holes – The basics
As we enter the period of the Solar Cycle when the Sun’s activity wanes, it might be expected that Auroral displays become fewer and further between. After all, it is solar activity some 93 million miles away that powers the Northern Lights in our night skies.
Fortunately, thanks to something called Coronal Holes that is not the case. Even during the deepest trough in solar activity, Coronal Holes ensure that the Northern Lights still appear regularly in the skies just north of the Arctic Circle or, what we call, The Aurora Zone.
Coronal Holes are regions of the Sun that are less dense and cooler than the plasma that surrounds them. Importantly, according to the Space Weather Prediction Center in Boulder, Colorado, they are regions of “open, unipolar magnetic fields” which allow High-Speed Streams of solar wind to escape into space.About Coronal Holes
Coronal Holes – Why are they important during Solar Minimum?
As we enter the less active part of the Sun’s activity cycle, events such as Coronal Mass Ejections which cause geographically widespread Auroras decline in number. This means that Aurora watchers are always on the look-out for the other source of Auroral activity, Coronal Holes.
Coronal Holes don’t throw out solar wind with the same ferocity as Coronal Mass Ejections but they do have the same effect of creating Auroras. The only real difference is that the lights appear in a much narrower geographical band. This band sits roughly between 66°N and 69°N and encompasses the northern areas of Finland, Norway, Sweden, and Iceland. This is the reason that the Northern Lights appear far more frequently in these regions than anywhere else and is exactly why we base our holidays here.
Click here to find out more about Solar Minimum.
Coronal Holes – A short-term tool for predicting Auroral activity?
The presence of a Coronal Hole on the surface of the Sun means that we can take an educated guess at when the Aurora might become more active. If that Coronal Hole is Earth-facing, then we can take a very reasonable stab at predicting an increase in geomagnetic activity.
Depending on the velocity of the Coronal Hole High-Speed Stream (CH HSS), the solar wind generally takes two to four days to reach our atmosphere. The images below certainly suggest that there is a direct relationship between Coronal Holes and the Kp index of geomagnetic activity.
On 04 October 2018, the large coronal hole on the surface of the sun was Earth-facing and allowed High-Speed Streams of solar wind to escape into space. Three days later, that same increased solar wind buffeted our atmosphere causing a marked increase in geomagnetic activity.
Coronal Holes – Persistence often pays
One of the great advantages of Coronal Holes - from an Aurora hunting perspective - is that they can remain on the surface of the Sun for several rotations. Coronal Mass Ejections are one-off events but a Coronal Hole can produce Auroral displays for two, three, four, five even six rotations before it eventually dissipates.
This means that we can also use Coronal Holes as a longer-term predictive tool. Admittedly, this is a far less accurate process because it takes 27 days for a Coronal Hole to come around again and, during that time, it can reduce considerably or disappear completely. Alternatively, it can get bigger and the solar winds emanating from it can get stronger.
Hence, if we see a large Earth-facing Coronal Hole we will be marking our calendars for its reappearance.
We can best illustrate this with the images below. The Coronal Hole in question first came to our attention on 12 August 18 and we were watching for the next rotation as it looked to have considerable potential. Sure enough, it reappeared in an Earth-facing position on 07 September 18 and resulted in a higher level of geomagnetic activity a few days later, demonstrated in the visual.
Polar and Equatorial Coronal Holes
Coronal Holes can occur anywhere on the surface of the Sun but are more prevalent at the poles. These Polar Coronal Holes do not release High-Speed Streams and are of little consequence to Aurora hunters. However, they often migrate north or south, sometimes as far as the equatorial regions and this changes everything.
This is illustrated if we look at the genesis of the Coronal Hole we have followed throughout this article. We first see it as the dark region around the Sun’s North Pole in the image below from 14 July 18.
At this point, it is of very little interest from an Aurora hunting point of view as any escaping solar wind will probably lack the sort of velocity required to create Auroras. Nor are they likely to be directed towards our planet.
14 July 2018 - However, a few days later on 19 July 18 as the Sun rotates, we can see the Coronal Hole has started to migrate southwards. All of a sudden, this is of interest because it has the potential to position itself continentally and release faster streams by the time it next faces the Earth.
19 July 2018 - One full rotation later and bingo! We see the emergence of a fully-fledged trans-continental Coronal Hole which is releasing earthbound High Speed Streams of solar wind.
12 August 2018 - Unfortunately, the Midnight Sun in Northern Scandinavia rendered any Auroral displays invisible to our still-hibernating photographers. They had to wait for one more full solar rotation to make the most of this beauty. Nevertheless, Antti Pietikainen found the perfect vantage point on 10 September 2018!
Suffice to say that Continental and Equatorial Coronal Holes release High Speed Streams of solar wind. When that wind is directed towards Earth, we will often see the Northern Lights a few days later.
Coronal Holes – The key points
- Coronal Holes are darker, cooler and less dense regions on the Sun’s surface
- Coronal Holes are the main source of Auroras when Solar activity is low
- Coronal Holes generally result in Auroras in a band between 66°N and 69°N
- Coronal holes occur anywhere on the surface of the Sun
- Continental and Equatorial Coronal Holes release High Speed Streams of solar wind
- Geomagnetic activity in our atmosphere increases a few days after an Earth-facing Coronal Hole appears on the surface of the Sun
- Coronal Holes are more persistent and stable than more explosive but one-off Coronal Mass Ejections. This means that they can reappear several times as it rotates back towards the Earth every 27 days.
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