The aurora is a stunning celestial event associated with Earth’s polar regions. Recently Ireland has been witness to this beautiful phenomenon.
If you are standing looking up at the sky in Canada, Alaska or in Norway on a clear dark night you may see a bright reddish or greenish band of light that stretches from east to west across the sky. This amazing natural light show has fascinated mankind for millennia and people will even travel thousands of miles just to catch a glimpse of it. Those living north of the Arctic Circle have a front row seat to this phenomenon, which is of course the aurora. An aurora is a natural display of light in the sky that can be seen with the unaided eye only at night. In the northern hemisphere it is known as aurora borealis (the Northern Lights). Its name comes from the Roman goddess of dawn, Aurora, and the Greek name for north wind, Boreas. In the southern hemisphere this spectacle is known as aurora australis (the Southern Lights). The word australis in Latin means “of the south”.
Aurorae are the most visible effect of the Sun’s activity on the Earth’s atmosphere, and are evidence that the Sun and Earth are connected by more than just sunlight. The occurrence and frequency of the Northern Lights depends on the activity of the Sun, which has an eleven year cycle. The displays are linked with the solar wind, a continuous flow of electrically charged particles from the Sun. These particles travel through space and if they reach the Earth’s magnetic field they travel towards the magnetic poles (the North and South Poles) colliding with oxygen and nitrogen molecules along the way. When a large number of these fast-moving collisions occur, it produces enough light for the eye to detect, and appear as coloured dancing streaks or arcs of light across the sky.
Aurorae are more frequent and brighter during an intense phase of the solar cycle when coronal mass ejections or solar flares increase the force of the solar wind. The next solar maximum is approaching, so in 2012 opportunities to see aurorae outside their normal range, possibly even from Ireland, should be more common.
The aurora can be seen in a multitude of colours from reds and greens to purples and blues. The colour is dependent on whether it is molecules of oxygen or nitrogen that are being excited. Oxygen emits a green or red light and these are the most familiar colours of the aurora. Nitrogen creates a display of blue light and if these gases blend then we see a display of purples, pinks and even white light. As well as the unique colours, the aurora also has unique shapes. The shape can look like a curtain of colour which seems to bend and bounce in the sky. Undulating ribbons of light may shimmer in the sky for hours with the altitude of its lower edge 100 to 112 km (60 to 70 miles) above the earth. Or the aurora may last 10-15 minutes, twisting and turning in patterns called “rayed bands”, then whirling into a giant green corona in which rays appear to flare in all directions from a central point, and finally fade away.
The oldest recorded sighting of the aurora was in 2600 BC in China. Throughout history many cultures have had their own interpretations of what these strange lights in the sky are. The eruptions of colour have driven folklore, created mythological creatures and have influenced the course of history, religion and even art! The Finnish name for the Northern Lights is ‘revontulet’ meaning ‘fox fires’, which comes from an old tale where a fox runs across the mountains beating his tail against the snow, sending sparks into the sky. For the Vikings the appearance of the aurora was a reflection of ghosts whilst the Germans called them “heaven light”. The Inuit people believed that the strange sky visions were walrus spirits playing with human skulls or were the flaming torches carried by departed souls guiding travellers to the afterlife. In central and southern Europe the light display is rarely visible and the main colour of the lights is strong red which was looked upon as a sign of forthcoming war or other atrocities. The Native American Tlingit in Alaska viewed the eerie sight as the dancing spirits of the deceased.
The Northern Lights are at their most dazzling from December to March when nights are longest and the sky is darkest. The best time to observe is the hours right before and after midnight. Usually an east to west curving arch first forms in the sky after which the lights begin to shift like ribbons dancing in the wind. If you are lucky the play of lights will end with a breathtakingly beautiful corona. A corona is the most impressive shape formed by the Northern Lights: there is a clear centre point from which rays form a crown shape above the viewer ( Editor’s note: ” I have been lucky enough to see this for myself, and was one of the most amazing sights I have ever witnessed!”) As auroral activity increases, the aurora not only increases in brightness, but it also tends to move further towards the equator. People in the northern United States and northern Europe may see the aurora a few times in a decade, while people in southern Europe, the southern United States, and even Mexico, may see the aurora only once in a lifetime. In fact Northern Ireland last experienced the northern lights back in 2005!
To learn more about this marvel NASA launched the Polar spacecraft in 1996 to obtain data from the Earth’s Poles. The spacecraft collected images of the aurora in action and studies how the solar winds from the Sun interact with the Earth. Using this data perhaps we can learn more about these events as we still have some unanswered questions such as why there are different shapes and does the display of lights create any sound? One day we will have the answers, but for now we can sit back and enjoy the show as the aurora is most definitely a truly awesome phenomenon, and is just one more example of the majestic beauty of our Universe!
If this is has whetted your appetite to see an aurora, you can come to Armagh and do just that. Over seven months of sub-zero evenings in the Arctic Circle, Terence Murtagh, a former Director of Armagh Planetarium, captured spectacular timelapse images of the Aurora Borealis. For the first time the aurora can be shown as it was meant to be experienced, as a display that covers the entire sky. Armagh Planetarium’s new immersive show Experience the Aurora shares the science behind the aurora while giving the audience the opportunity to share in one of nature’s great spectacles.
(Update: since this article was originally published the aurora has indeed been seen from Northern Ireland on 22 and 24 January 2012. I had added some material to Sinead’s original text reflecting my scepticism that we were going to see it, but I am now delighted to have been proven wrong! – CJ)
(Article by Sinead McNichol)