13 January 2014 16:59

Where should I go to see the northern lights? It’s a question I get asked a lot, usually just after I say that I study the physics of the aurora borealis. More often than not, it is followed by "When’s the best time to see them?"

It's not surprising. The northern lights are one of nature’s most beautiful spectacles. A whole tourist industry has sprung up to help aurora chasers to try and catch a glimpse of these ghostly lights in the Arctic skies. So where is the best place to see them and when is the best time?

Aurorae are caused by the interaction of the Earth’s magnetic field with the solar wind, the stream of electrically charged particles constantly emitted by the Sun. Because the solar wind is electrically conductive, it drags remnants of the Sun’s magnetic field out into interplanetary space. When it arrives at the Earth, the magnetic field in the solar wind can couple with the Earth’s own magnetic field. Ions and electrons in the region of near-Earth space known as the magnetosphere are energised by this coupling and some, guided by the shape of the terrestrial magnetic field, are funnelled down towards the magnetic poles. Collisions with atoms in the upper atmosphere cause it to glow, with the characteristic green and red colours of the aurora emitted by oxygen atoms at altitudes between 100-150 km above the ground.

This process generates aurorae in crown-like ovals surrounding the magnetic poles. Although the solar wind is continuous, changes in the efficiency of the solar wind-magnetosphere coupling process modulates the auroral activity. Nevertheless, residents of northern Scandinavia, Iceland, northern Canada and Alaska regularly witness stunning natural light shows, weather and darkness permitting. But what about closer to home?

During periods of increased solar activity, the aurora can occasionally be seen at much lower latitudes than usual, but a particular chain of events need to occur. Typically, it starts with an eruption of material from the Sun known as a coronal mass ejection (CME). An average CME can include a billion tonnes of solar material moving at a million miles an hour, crossing the interplanetary space between the Sun and Earth in just a couple of days. If the magnetic field within the CME is aligned anti-parallel to the Earth’s, then the CME’s energy and momentum can efficiently couple into the magnetosphere, triggering a geomagnetic storm. This heightened geomagnetic activity causes the auroral oval to expand, pushing aurorae southward. If the skies are clear and the geomagnetic storm coincides with the hours of darkness, mid-latitude observers including those in the UK can be treated to auroral displays.

So why are auroral displays from the UK so hard to predict? Although they can occur at any time, the CMEs that cause them are most common during solar maximum, the peak of solar activity that occurs every 11 years. They can be fired outwards from the Sun in almost any direction, so the geometry has to be just right. If it is Earth-bound, predicting a CME’s time of arrival at Earth is not straightforward. Uncertainties of around 10 hours are not unusual meaning that a CME could arrive and trigger aurorae during daylight instead of darkness. The orientation of the magnetic field inside the CME is crucial. If it is aligned parallel to the Earth’s magnetic field, then the coupling is weak whereas if it is antiparallel the coupling is strong. Unfortunately, this crucial parameter cannot be measured until the CME passes the ACE spacecraft located approximately 1.5 million kilometres upstream of the Earth, only one hour before the CME arrives at the Earth. Finally, even if all the other factors are favourable, cloudy skies or light pollution can put pay to any aurora being seen from the ground.

We know from historical data that the “perfect storm” scenario, where each element of the chain is maximally geoeffective, can push aurorae as far south as the Caribbean, but these occur less than once each century. Geomagnetic storms that result in aurorae visible from the south coast of England are more common, perhaps occurring once every twenty years or so. Set against this background, auroral displays visible from northern Scotland are relatively common, occurring a few times a year.

So if you want to try and see aurorae from the UK, you might want something to tip the odds in your favour. Fortunately, Lancaster University’s free AuroraWatch UK service does exactly that, alerting users to heightened activity in the UK via email and social media. When I’m asked what’s the best way to see the northern lights from the UK, it’s what I recommend. That, patience and a bit of luck.

What do you think? Share your comments with us below.

Professor Jim Wild teaches on our BSc Physics, Astrophysics and Cosmology programme.


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