Spectacular ‘dancing’ of the Northern Lights show
A FRIEND of mine with a small reserve on Orkney phoned me the other night to describe to me the North Lights he was watching. I wasn’t jealous at all, not much, and it reminded me of a time thirty years ago when I was on the hills above Inverness minding my own business after a day tracking the elusive Scottish wild cat; the night was closing in, and after bagging, what I hoped was, at least one picture of this enigmatic feline, I was keen to land back in a Loch Ness side pub, the Dores Inn, in the village of Dores on the Eastern shore of the peat darkened loch of mystery.
It was a cold night, the sky cloudless, and every star and planet shining bright; however, as amazing as the sight was, it did not prepare me for what was to come.
Sheets of shimmering green and red lights, with purples, yellows, oranges and ethereal threads of neon-green-filled sky. It was wonderful and a nightmare all at once, the former for all the obvious and life-affirming reasons, the latter, because I’d run out of film in those far off pre-digital days.
The Northern Lights, or Aurora Borealis, are created by charged particles colliding with the Earth’s upper atmosphere. The particles cause a change in atoms of the upper atmosphere that release light as they return to their normal state.
Auroras are now known to be caused by the collision of charged particles (e.g. electrons), found in the magnetosphere, with atoms in the Earth’s upper atmosphere (at altitudes above 80 km). These charged particles are typically energized to levels between one thousand and 15 thousand electron-volts and as they collide with atoms of gases in the atmosphere, the atoms become energized. Shortly afterwards, the atoms emit their gained energy as light.
Light emitted by the Aurora tends to be dominated by emissions from atomic oxygen, resulting in a greenish glow, especially at lower energy levels, and at higher altitudes, the dark-red glow. Both of these represent transitions of electrons of atomic oxygen that, in absence of newer collisions, persist for a long time and account for the slow brightening and fading of auroral rays. Many other colours, especially those emitted by atomic and molecular nitrogen (blue and purple, respectively) - can also be observed. These, however, vary much faster and reveal the true dynamic nature of auroras.
As well as visible light, auroras emit infra-red and ultraviolet rays as well as X-rays. While the visible light emissions of auroras can easily be seen on Earth, the UV and X-ray emissions are best seen from space, as the Earth’s atmosphere tends to absorb and rarefy these emissions.
Typically the aurora appears either as a diffuse glow or as “curtains” that approximately extend in the east-west direction. At some times, they form “quiet arcs”; at others “active aurora”, they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that aurora is shaped by the earth’s magnetic field. Indeed, satellites show auroral electrons to be guided by magnetic field lines, spiralling around them while moving earthwards. The earth is constantly immersed in the solar wind, a rarefied flow of hot plasma gas of free electrons and positive ions emitted by the sun in all directions, a result of the million-degree heat of the sun’s outermost layer, the solar corona.
Okay that’s the science, here’s the human side and my favourite literary reference to the Lights:
“When I was a lad, a tiny wee lad,
My mother said to me, ‘Come see the Northern Lights my boy,
They’re bright as they can be!’
She called them the heavenly dancers,
Merry dancers in the sky, I’ll never forget, that wonderful sight
They made the heavens bright!”
So goes the chorus of one of Scotland’s best-loved songs, The Northern Lights of Old Aberdeen.
Dancing was what I felt like doing after my spectacular light show all those years ago, and as it happens, I’m not really bothered to this day that I had no film, and besides, I did get one great shot of the wildcat known as ‘Big Tom’, and yes you guessed correctly, my celebration later that night involved a dram or two. ribblediscoverycentre