How aurorae happen

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On the night of Monday March 13, 1989, New Zeal­anders were treated to a spectacular display of the Aurora Australis or “south­ern lights”.

Many parts of Australia also witnessed this breath­taking display which resulted from a large solar flare occurring a few days earlier.

Solar flares, which last for about 20 minutes, are caused when the normally 5000°C gases above the Sun’s surface are suddenly heated to 10,000°C.

At 5.38am NZST (New Zealand Standard Time) on March 10, a flare some 36 times the size of the Earth erupted and shot 100,000km into space. Electrically charged particles (electrons and protons) accelerated by the flare travelled at 500km per second, arriving at the Earth in a little over three days to cause the aurora.

Earth has a strong magnetic field by plane­tary standards, and this is usually sufficient to deflect most electrical particles streaming from the Sun. However, under flare conditions these particles can collect behind the Earth and a sort of magnetic explosion takes place, hurling some of the collected particles into deep space and squeezing the rest back against the Earth to spiral down the magnetic field lines. These particles collide with the upper atmosphere and cause it to glow from a height of 100 to 200km – an effect similar to the glowing gas of a neon sign.

This glow is the aurora. It spreads in a large ring centred on the magnetic pole, and in our part of the southern hemisphere normally reaches only as far north as the sub-antarctic islands.

During a strong influx of electrical particles from the Sun the ring expands, and on rare occasions, such as the March 13 aurora, it can reach much further north. That night the aurora was observed from at least as far north as Exmouth in Western Australia, at latitude 21.8°.

Invercargill, in the south of the South Island, is well placed for observing aurorae. On the evening of March 13, even before the twilight of the setting sun had disappeared, an eerie, pale green glow was evident in the south, with areas of cloud low on the horizon appearing dark against it. For the next hour this glow gradually filled more and more of the sky, finally stretching past the zenith (the point directly above an observer’s head).

Simultaneously, a faint, pale green rainbow-shaped arc slowly moved higher and higher in the sky until by 10.00pm NZST it stretched west to east and passed through the zenith. Soon after this two broad rays from the western horizon rose high into the sky – one pale green, the other bright red. The red one moved up the arc, and within 15 minutes many more red and cream-coloured rays were reach­ing up to near the zenith from all parts of the sky. At this stage the aurora was so bright it was casting a faint shadow.

Just as when you look up into rain, and the falling drops appear to radiate from a point, so too these colourful auroral rays appear to radiate from a point near overhead, creating a feature referred to as a corona. The radiant point is known as the observer’s “magnetic zenith”, and from Inver­cargill this is slightly north east of the true zenith.

By 11.00pm the aurora had faded back to a glow, but the early morning hours of March 14 were witness to resurgences of the bright red colouring.

Over the next two or three years, more large, bright aurorae are likely to be seen, particularly from southern New Zealand and Australia. This is because the Sun is approaching what is called its “maxi­mum” of activity. For reasons not yet understood, the Sun has quiet periods, where the flares are few and weak and the sunspot numbers are low, and active periods (such as now), when it develops extremely energetic flares and numerous sunspots that are often very large.

These periods of mini­mum and maximum activity, the “solar cycle”, occur on an 11-year basis. Not all maxima are as active as others, but the present one appears set to break all records. It rose from a 1986 September minimum very rapidly and is essentially a year ahead of schedule. Sunspots are a useful indicator of the Sun’s activity and many amateur astronomers contribute to our knowl­edge by counting them daily.

Sunspots are regions on the Sun’s surface, often very much larger than the Earth, where gases rise, cool, and sink back down. In cooling to 3000°C they appear dark against the 5500°C surface. Some of the largest spots or groups of spots can be seen with the naked eye, although they must be viewed through a dark piece of glass or a welder’s helmet to prevent eye damage.

Never look at the Sun through binoculars or telescopes, as this will result in instant blindness!

An enormous group of spots was associated with the March aurora-produc­ing flares, and was fol­lowed by observers for two weeks as the Sun’s rotation carried it across the surface facing us. This region was very active, producing flares approximately once a day during the time it was visible.

It only needs this region (which may survive several 28-day solar rotations), or a similar one, producing more flares in our direc­tion, to create another spectacular aurora. It is therefore worthwhile keeping a close watch on this winter’s night skies.

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