Shimmering brushstrokes float across the sky

When the moon rises over the ridge these dark nights, its radiance precedes it as a brilliant glow beyond the profile of the hill. Stepping out from the house the other evening, soon after 9 p.m., I registered this familiar and pleasing effect, then did a double-take. We had seen the moon much earlier: a cold new crescent in transit at the other side of a cloudless sky.

Even as we watched, standing at the gate, a wavering in the light, an upward streaming, lifted our eyes to a banner of colour, a deep rose, almost crimson, where no afterglow of sunset could possibly be, and then to other such shimmering brushstrokes floating above the hill. This could be only one phenomenon - something I had waited a lifetime to see: nature's own heavenly light show, aurora borealis.

Through the next half-hour the lights expanded to a great, high arch in the northern sky, filling my vision from side to side, and from almost overhead to the fog-banks low on the horizon. Islands, headlands, the landward hills were embraced in this thrilling proscenium.

Within its arc, slowly shifting veils of a gleaming greeny-white were blotting out the stars, clouds of crimson dissolved and reformed above faint and flickering rays that rose as if from distant, invisible searchlights.

"The motions," as Barry Lopez wrote in Arctic Dreams, "were like a t'ai chi exercise: graceful, inward-turning and protracted." We phoned friends in the next townland who were having a party and beseeched them to get everybody out there, this minute, and switch off all the lights. They called back later at the final fading, awed and full of questions. I had a few myself, not least why, after 20-odd years of living at a perfect vantage point beneath a pristine night sky, we should suddenly be visited with such a spectacle.

Ethna had watched the northern lights from Killala in north Mayo in the 1950s. A friend in Sligo had seen them again, in the 1970s. The Irish phrase na gealβin tuaidh, the flash of light in the north, speaks for long acquaintance. All this discounts the notion, often found in books, that this famously Arctic (and Antarctic) phenomenon petered out around the latitude of 57 degrees north, or roughly a line through Scotland's Hebrides.

Nonetheless, its periodic visibility in Ireland suggests a special intensity of effect, at long intervals. So why on the night of October 21st? The aurora borealis and australis (there are two magnetic poles) wax and wane in 11-year cycles that match fluctuations in the state of the sun's nuclear bonfire. A US science website, http://www.spaceweather .com, offers popular bulletins on sunspots and the flaring eruptions that hurl streams of electromagnetic particles in a solar wind towards Earth.

On October 21st, it noted, "a coronal mass ejection swept past our planet and ignited a long-lasting geomagnetic storm. Some watchers at high latitudes have enjoyed a vivid display of northern lights."

Some, indeed, had their cameras and tripods at the ready, and a dozen brilliant photographs in the website's gallery show how the lights appeared in an arc from northern Europe to North America. "The most beautiful I have ever seen," judged one Norwegian, for whom the aurora is a regular winter spectacle. At Nice, in the south of France - a mere 44 degrees north - "pulsating red and pink columns rose from the ground". In Germany, the lights were so bright they could be photographed through clouds.

It is easiest to imagine the aurora as resulting from a massive short-circuit as the rushing wind of charged solar particles reaches the outermost layer of Earth's atmosphere, the ionosphere. As the plasma curls around the planet, it flows and eddies along the force lines of the Earth's magnetic field, curving down at the poles in a funnel of electric current.

The auroral display is formed in the ionosphere, where the energy in the stream of particles excites electrons in our atmospheric gases. In billions of quantum leaps, kinetic energy becomes light, molecules of oxygen tinting it green and nitrogen flushing it red. The display remains high above the Earth in a luminous wall, but it can seem to touch the surface, such is our difficulty in judging where things are in space.

It was as well to know a little about this, as we marvelled at the ghostly transformations of the sky. How enthralling, or ominous, according to one's mythology or temperament, they must have seemed in medieval Europe: a supernatural apparition, either way.

Today, of course, in any part of Europe where the reach of the aurora is intermittent or rare, it is likely to be missed altogether by the great majority of people, such is the haze of light pollution that hides the night sky. What hope of distinguishing, in Dublin, the crimson clouds of excited nitrogen from mere reflections of metropolitan neon?

An RT╔ radio correspondent, composing a personal colour-piece from a front-line somewhere in Afghanistan, described her wonder at the dazzling stars in a night far removed from any streetlights. Never before, it seemed, had she gazed so raptly into the twinkling infinities of the Milky Way. As wasteful, heedless light is spilled across Ireland, the beautiful birthright of space and stars is shut out from the human experience.

A couple of summers ago, I wrote excitedly about seeing the "green flash", a rarely observed and momentary flare of emerald light at the upper rim of the setting sun. It is a beautiful optical phenomenon, born of refraction and the scattering of sunlight - a chance curiosity that demands, in Brendan McWilliams's phrase, "a very rare, almost freak, thermal structure of the atmosphere". That I happened to be looking at the right second was just a matter of luck.

Walking out of the house as the northern lights began was another, memorable stroke of fortune, but one immeasurably heightened by a clear, dark sky.

Chance is one thing, opportunity another.

Michael Viney welcomes nature observations sent to him at Thallabawn, Carrowniskey PO, Westport, Co Mayo. Observations sent by e-mail should be accompanied by a postal address.

E-mail: viney@anu.ie