Aurora Borealis: A Guide
By Brendan Alexander
As darkness descended upon Donegal and Northern Europe on the evening of 22nd of January 2012, the northern horizon began to shine with a colourless glow. As the night progressed, the glow developed into curtains of shimmering light and the more keen eyes could even detect the slightest hint of green at the base of the eerie glow. This was a rare display of the Northern Lights or the Aurora Borealis. The Aurora Borealis have long been the subject of mystery and folklore, striking wonderment and fear into the hearts of the planet’s most northerly inhabitants.
Folklore of the Northern Lights
The Finnish call the Northern Lights “Revonulet”. Directly translated this means the “Fox Fires” and originates from an ancient Finnish myth in which the lights were believed to be caused by a magical fox majestically sweeping his trail across the snow, spraying it into the sky. The Lapps of northern Finland, Sweden and Norway attributed the lights to the souls of the departed. When the souls energetically blazed across the skies, the natives behaved solemnly to respect the souls because they feared that doing otherwise would invite bad fortune. The Algonquin Indians believed that Nanahbozho, their creator, travelled to the far north after he finished creating the Earth. From there, they believed that he built great fires which reflected southwards , to remind those he created of his lasting love. Closer to home, the Scots often referred to these majestic lights of the north as “the merry dancers” owing to the Norwegian folklore in which the lights were believed to be spirits of old maids dancing and waving in the sky.
Science of the Aurora Borealis
The familiar name of the Aurora Borealis was attributed to the northern lights in 1621 by French scientist, Pierre Gassendi. They were named after the Roman goddess of the dawn, Aurora and the god of the north wind which in Latin is Boreas. The Aurora Borealis is caused by a complex string of events in which the eerie lights above the snow-draped landscapes of the arctic can be traced back to the surface of the Sun. The light from the Aurora is light that is actually emitted from atomic collisions in the Earth’s upper atmosphere. Although the Aurora appears colourless to the naked eye, for the most part the green that is commonly picked up in long exposure photographs originates from atomic oxygen in the Earth’s atmosphere colliding with an incoming particle, typically an electron. The collision transfers energy to the oxygen atom, which is then released again in the form of visible light. Light at a green wavelength is characteristic of atomic oxygen. The combined effect of billions of these particle collisions in the Earth’s atmosphere gives rise to the Auroral glow. The Earth’s magnetic field is not dissimilar to a massive particle accelerator, catching particles passing in space and hurling them into atoms in the Earth’s atmosphere. The out flow of accelerated particles from the Sun results in Aurora displays on the majority of dark nights in the artic. However, for the aurora to spill south into Irish skies, something much more spectacular must occur. For Aurora displays to occur in Irish skies, the Sun must blast material from its surface towards Earth in an event called a coronal mass ejection (CME). This sends a mass of charged solar particles hurtling into space and sometimes towards the Earth. This cloud of superheated plasma can sometimes contain billions of tonnes of material and travel at speeds of up to one million kilometres per hour. When these charged particles reach the Earth’s upper atmosphere, they collide with neutral atoms resulting in the release of photons (light particles). The combined effect of many photons being released from many atoms resulted in the Aurora displays that were visible from Donegal last month. Such events are quite rare though and the fact that clear skies are required to see an Aurora display means that displays over Donegal and Ireland are rare occurrences. Nonetheless, Northern Light displays are spectacular sights on the sporadic occasions that they are visible from Ireland. When a CME delivers a blow to Earth, it distorts the Earth’s magnetic field so much that the Auroral oval, under which the Aurora can be seen (see picture) slips further south towards the equator and on extreme events, it can position itself over Ireland.
Such an event coincided with clear skies over Donegal last month and in March 2011 before that. On both occasions, the interplanetary magnetic field (IMF), a very important component was in a favourable position. On these occasions the IMF (also referred to as the Bz Component) was tilted southwards, strengthening the magnetic pathways between the Earth and the Sun. This, coupled with the arrival of a CME or increased solar wind speed, can result in Aurora at much lower latitudes than normal. On the 22nd of January 2012, from Fanad head and, indeed, across the northern half of the country, an intense white light not unlike twilight glow was seen between Sunset and Sunrise. It soon became apparent that this was not the effect of the Sun setting minutes before but, rather, the effect of a violent eruption from the Sun’s surface days earlier. Soon a feature called Auroral rays became visible. These are shafts of more intense light emanating from the base of the Auroral glow which shimmer slowly cross the base of the Aurora, giving rise to the infamous curtain effect. These are created when the Aurora traces the Earth’s magnetic field lines and appear as bright shooting beams of light. All this took place against the backdrop of twinkling stars. The steady orange glow of street lights illuminating the clouds is often mistaken for the Northern Lights. A simple way to eliminate this mistake is to check for the presence of stars behind the glow, as starlight will not shine through cloud but will penetrate Aurora displays. To the naked eye, Aurora displays are whitish with only very subtle hints of colour, mostly green. An intense, steady, orange starless glow is a sure sign of light pollution, not Aurora.
All through 2012, 2013 and 2014 there is an increased chance of Northern Light displays over more southern latitudes like Donegal and the rest of Ireland due to the Sun reaching its solar maximum. The Sun runs in cycles of activity over an 11 year period and is predicted to reach solar maximum (most active) in the years ahead. Around the time of solar maximum, violent eruptions like CMEs on the Sun are more frequent. This is good news for Aurora hunters in Donegal and the rest of Ireland, as the Earth experiences increased Auroral activity either side of solar maximum. The Aurora is typically more active around the time of the equinoxes in March and September. However, the long winter nights also offer ample dark hours for viewing any possible displays. As the displays are faint, Moonlight does interfere, so a clear, Moonless night is ideal to view the Aurora. Most importantly, anyone hoping to witness possible displays should get to a very dark location, away from all light pollution with a clear view of the northern horizon.
Although it is true that the further north you are the better for viewing the Northern Lights, it is not advisable to travel large distances to see a potential Aurora display because there is no guarantee that the Aurora will be visible as cloud and other factors often interfere. A potential display should be visible throughout the country due to the small geographical size of Ireland. A much more frequent inhibitor of a potential Irish display is cloud and, unfortunately, Donegal typically has some of the cloudiest skies in the country.