Aurora Borealis in Norway – Why do high latitudes get most of the aurora?
Aurora borealis in Norway: When the word aurora comes to mind, people usually picture cold snowy places. On rare occasions, the lights can appear further equatorward, in places you would never have imagined! But why do high latitudes get most of it anyway?
The aurora comes from the Sun
When the Sun has no major activity, also called solar events, we say it is solar quiet. All our star does is to spew a background stream of particles called solar wind. This solar wind is always there and is the base for spawning aurora on Earth. Thus solar quiet conditions will give quiet auroral conditions back on Earth.
In contrast the Sun sometimes hosts major solar events that come and break the solar quiet. These events happen more often during solar maximum. We discern 3 major events: Solar Flares, Coronal Holes (CH) and Coronal Mass Ejections (CME). All three participate in enhancing the background solar wind, sometimes blowing huge solar storms out into the interplanetary space. Solar events create very active- sometimes extreme, auroral conditions back on Earth.
Low activity and high activity aurorae
Back on Earth the aurora is produced by very intricate processes. People usually don’t know that there are two distinct kinds of aurora situations.
When there is no major solar event, the solar wind is rather quiet. There might be some sporadic pockets of enhanced solar wind here and there. During this time there is what we call ‘background aurora’. This kind of aurora is produced only at high geomagnetic latitude by a process called magnetic reconnection. It happens all the time and there doesn’t need to be a major input from the Sun for the aurora to appear in the night sky. That’s why high latitudes get aurora more frequently than lower latitudes.
When a rare major solar hurling huge numbers of particles towards us, it interacts with our magnetic field so violently that not only high latitudes get aurora via the same process as seen before (albeit more vigorous and long-lasting) but also lower latitudes. The solar storm can penetrate so deep in our magnetic shield that it rattles the inner field lines producing long-lasting, colorful and very active aurorae that can be seen very far from the poles. We call these events geomagnetic storms.
The substorm: an isolated phenomenon in three phases
When the geomagnetic activity is at its lowest, high latitudes still get to see some aurora. However it will likely remain as very faint and slow-evolving bands, mostly on the horizon as seen in the panoramic picture below.
If the activity gets just slightly higher, the aurora usually starts growing. It is the first phase or onset of what we call a substorm. ‘Growth’ is a term used in the aurora jargon and it means three things that go hand in hand. First, it means the aurora bands tend to migrate away from the pole and towards the equator. Secondly, it also means that the aurora gets brighter. Finally the aurora tends to describe more structures like pillars. If the geomagnetic activity suddenly ceases to be favorable, the aurora no longer grows and ‘recedes’ back. It sometimes almost completely disappears! In the picture below you can see the result of the growing phase. It was taken on Senja island, 5 minutes from the observatory. You are not looking due north, as in the previous photograph. This view is due south! The growth phase had the aurora migrate all the way to the other horizon.
If the conditions remain favorable, we enter the second phase of a substorm: the expansion phase. The band starts moving faster, getting more structure and getting brighter.
It tends to migrate back towards the poles. For very high latitudes, it usually means that the band come back overhead. Meanwhile it continues to get brighter, more colorful and defined.
When you see this happening, you can be almost certain that you will get a brilliant aurora show. From that point on, the aurora starts getting extremely bright, dancing very fast over your head and producing colors visible with naked eyes: We call it a corona.
It’s the peak of a substorm. It’s like the aurora is exploding into thousands of bright colors. It dances very quickly all around and can sometimes fill up the whole sky, resembling a crown, hence the name ‘Corona’. In the process the thin band expanded into a wide aurora blanket, which is why we call it the expansion phase. While the growth phase can take up to an hour, the expansion phase usually is much quicker. It takes anywhere from one to about 20 minutes. Coronas are the most impressive part of a substorm and are the ultimate goal of any aurora chaser.
Once the sky intensely lights up for a little while, it fades out into the third phase: the recovery phase. This is a phase that can last for several hours. After an intense ‘explosion’, the sky faintly flickers. What’s left is a diffuse aurora that is often colorless with the naked eye. We call it pulsating aurora.
Substorms are a quite isolated phenomenon. They happen all the time at high latitudes even when the geomagnetic activity is rather low. We get them all the time at the aurora borealis observatory. Our location according to the auroral oval means that most of the time the aurora explodes right overhead. The last season alone, when had about 30 auroral coronas! One more reason to book with us for next season!
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