The beautiful blaze of the Northern Lights, or Aurora Borealis, is caused when material thrown off the surface of the sun collides with the atmosphere of the Earth. Thus, by following events on the sun and the velocities of the gaseous matter being thrown off its surface, we can predict the appearance of the Northern Lights with a fair degree of accuracy — certainly enough to meet the needs of the average observer of the night sky. These predictions and observations are collectively referred to in the style of weather forecasting as ‘space weather’.
The aurorae appear over the Earth’s polar regions in what are known as the auroral ovals; in the northern hemisphere the auroral oval bulges that much further to the south, the stronger the solar wind is at any given moment. The oval normally extends over northern Finland and Scandinavia, the whole of Canada and the northern USA, Alaska and Siberia. In the event of a solar storm, it may reach as far south as the skies over central Europe. Because the oval does not extend symmetrically around the Earth's rotational axis, each degree of the Earth’s longitude rotates deeper into the oval once every 24 hours; in the case of Finland this rotation means the best time for viewing the Northern Lights is around 10.30 in the evening (Standard Time). On the other hand, it is always worth bearing in mind that a solar storm can appear at any time of the day or night, and hunters of spectacular shows would therefore be well advised to concentrate on following the various types of forecasts and predictions which are published on the Internet.
A natural place to begin scanning for predictions of the Northern Lights is the website Today’s Space Weather, which provides an estimate in easily understood layman’s language of the situation over the next three days. Another site, SpaceWeather.com, which is aimed specifically at the general public and provides an excellent commentary on near space phenomena, includes precise details of predicted sightings of the Northern Lights. Real-time information on disturbances in the Earth’s magnetic field, which are caused specifically by the aurora, is provided by the Sodankylä Geophysical Observatory Magnetogram. If the curves depicting the disturbances exhibit a sudden fluctuation of 1000nT (a smaller fluctuation will often be enough), the Northern Lights will probably be visible at that moment as far south as southern Finland. The best and most reliable prediction of all, however, is turning out to be the latest activity index prediction derived from the measurements taken by the Advanced Composition Explorer (ACE) satellite used in sensing the solar wind between the sun and the Earth. Its prediction Latest output (1 day) shows the predicted auroral situation 35-70 minutes ahead on a scale of 1-9. Experience has taught that an activity index value of 5 will often already mean a handsome display of the Northern Lights in southern Finland.
Despite the abundance of helpful predictions, it’s worth remembering that they may not always turn out to be correct, or that it may be daylight in Finland at just the moment the Northern Lights are illuminating the sky. Note too that the predictions nearly always use UT, or Universal Time, from which you can calculate Finnish Winter Time by adding on two hours, and Finnish Summer Time by adding on three. And if you can’t be bothered following the Internet, you can always time your evening walk for between ten o’clock and midnight, and set out for a spot where you can get a good view of the northern sky. The sun is currently going through an extremely active phase, which means frequent occurrences of the Northern Lights; it may therefore be that one evening nature will provide an unforgettable extravaganza of colour, which will by comparison reduce all the shopping centre laser shows in the world to the level of a pocket torch!