How does the aurora borealis work?

The astronomically inclined amongst you have no doubt been aware of the aurora borealis, or northern lights, happening across the UK recently. As the name suggests the northern lights usually only occur in the far northern, particularly in Scotland, and even then only periodically. But this week some dramatic auroral displays have been seen as far south even as Cambridgeshire, with some stunning images coming from the Fens just 10 minutes from where we’re sitting. So what forces are behind such a majestic phenomenon, and why were they so far south this time? To guide us through the furore of the aurorae is ex-NASA intern and guide at the Griffith Observatory Rosemary Williams…

The aurora borealis, or northern lights, are one of the most beautiful sights on Earth. Dancing patterns of greens and reds in the sky have inspired a millennia of folklore. The norse considered them to be earthly manifestations of their gods. Some indigenous groups considered them to be a bad omen, and were you to whistle underneath one you would be noticed and carried up into the sky.

The northern lights are typically found in the Arctic circle. This area doesn’t encompass much land, it’s too far north for even Iceland to be inside of it, which makes seeing this event a somewhat rare occurrence.

However, auroras over the past few days have been seen as far south as Cambridgeshire and Shropshire. So what causes an aurora to form, and why was this one so much more widespread than normal?
Auroras happen when the sun sends out a solar flare. This is a million mile an hour wind that is full of charged particles called ions. These ions hit Earth’s magnetic field and interact with the elements that are high up in the atmosphere. Earth’s magnetic field usually does a pretty good job of protecting us from these high energy particles, except at the poles. See our magnetic field is sort of apple shaped - the magnetic field turns in towards the Earth at each pole and can actually funnel these charged particles toward these northern and southern intersections. Auroras are pretty much a daily occurrence at our magnetic north and south poles.
These ions cause the electrons in these element’s outer layers to temporarily jump up to a more excited energy level. When these electrons return to a more stable state, they release energy in the form of visible light, anything from dark reds to light greens. Green and red light occur when low and high altitude oxygen interact with these solar particles, while blue and purple light occur when Nitrogen interacts with these particles.

But what was special about this one?

Well, the ions involved in this aurora left the sun via a ‘coronal hole’. This is different to a regular solar wind because it reaches out further into space and pushes the charged particles out at a higher speed. This means Earth gets hit with a greater amount of solar activity than usual. Not enough to send our satellites crashing down to the ground, but enough for ions to congregate in our atmosphere at a lower latitude.
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So that’s why the skies were red over Cambridge. And the news looks good for any keen aurora watchers, as we’re entering a time of high solar activity. So keep your eyes skywards, just remember not to whistle.