At the beginning of November, residents of Scotland and Northern England were able to view a dazzling light show in the sky: the Northern Lights. But what causes them and how can we predict when it will happen again? The Northern Lights are a natural phenomenon where brightly, coloured lights are seen across the night sky in the appearance of sheets or bands. They are generally seen close the magnetic poles in an area called the ‘auroral zone’. The best time to spot the auroras is when the Earth’s magnetic pole is between the sun and the location of the person observing. This is called magnetic midnight. The Northern lights are caused by gaseous particles in the Earth’s atmosphere, colliding with charged particles, released from the sun’s atmosphere. The charged particles are carried towards Earth by solar winds. The particles are deflected from the Earth’s magnetic field. However, at the poles, the field is weaker allowing a few particles to enter the atmosphere. Hence this is why auroras are more likely to be seen close the magnetic poles; making Iceland and Northern Scandinavia common destinations for travellers searching for the Northern Lights. The colours of the Northern Lights are dependent on the type of gas molecule involved in the collisions. Green is one of the most common colours seen and is caused by collisions of oxygen molecules, whereas blue or purple auroras are caused by nitrogen molecules. Why can the northern lights sometimes be seen in places further from the Earth’s poles e.g. the UK ? The answer is the spread of aurora oval due to ageomagnetic storm. Geomagnetic storms are more common after the maximum in the solar cycle, a repeating 11-year cycle. The most recent solar maximum was in 2013. The Northern Lights are notoriously unpredictable. There are many forecast apps available such as “My Aurora Forecast”. One of the best websites to check out when the auroras will be visible from where you are is the Aurora Service (www.aurora-service.eu/aurora- forecast/). The site gives the Kp value predicted for the next hour by using solar activity data obtained from a NASA spacecraft, ACE. The ACE orbits 1.5 million kilometres from Earth: the prime position to view the solar winds. A common way to represent geomagnetic activity is the Kp index. Magnetic observatories located all over the world use instruments to measure the largest magnetic change every three hours. The recorded data from all these observatories is averaged to generate Kp values, which range from 0 to 9. The larger the value the more active the Earth’s magnetic field is due to geomagnetic storms and the further the aurora oval spreads. If the Kp value is above 4, then it is storm-level geomagnetic activity. These Kp values are useful in predicting when auroras will be visible. To see the aurora from the UK, the Kp value would have to be at least 6.
To get a great show, the conditions are important. Clear nights with no clouds are best. It is also worth checking the moon cycle: the brightness of a full moon drowns out the lights of aurora.