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As natural phenomena go few come more spectacular or mysterious than the northern lights. PETER FROST dons his astronomer’s hat to reveal their provenance

They saw them in Scotland, in Northumberland, on the Isle of Man and as far south as north Norfolk. It was some of the best British sightings of the aurora borealis, the famous northern lights, in living memory.

Hundreds of people all over Britain braved the freezing late night and early mornings but declared the experience one well worth getting frozen for.

Those lucky enough to see them described spectacular waves, streaks or curtains of pale green and pink, but shades of red, yellow, blue and violet were also spotted.

It’s rare for northern lights to be seen from anywhere in Britain and when they are visible it is usually from Shetland, Orkney or the north of Scotland.

Last week however, good sightings could be had from all over the country as far south as Norfolk. These amazing multicoloured ethereal light displays are caused by collisions between electrically charged particles from the sun that enter into the earth’s atmosphere.

aurora_2117845b

They are more common much further north and British tourists normally need to take cruises or air holidays to northern latitudes if they want to see the amazing spectacle.

Polar lights — the aurora polaris — are a natural phenomenon found in both the northern and southern hemispheres. The northern versions are called aurora borealis while the southern lights aurora australis.

They were first named by two great early astronomers Pierre Gassendi and Galileo Galilei both of whom witnessed a spectacular display in September 1621. They jointly named the phenomena aurora borealis — the northern dawn.

Much earlier, a thousand years ago, Gregory of Tours, Gallo-Roman historian, scientist and later saint looked into the night sky over France and saw a light “… so bright that you might have thought that day was about to dawn.”

We now know the origin of the aurora starts on the surface of the sun when solar activity ejects a cloud of gas. If one of these reaches Earth it collides with its magnetic field two or three days after leaving the sun.

Our planet’s magnetic field is invisible but if it could be seen it would make Earth look like a comet with a long magnetic tail stretching a million miles behind us away from the sun.

When a coronal mass ejection (below) — as the stream of cloud of gas from the sun’s surface is more properly named — collides with the magnetic field it causes complex changes to happen to the magnetic tail region.

eruption-producing-a-coronal-mass-ejection (1)

These changes generate currents of charged particles, which then flow along lines of magnetic force towards the Earth’s poles.

The particles are boosted in energy in Earth’s upper atmosphere and when they collide with oxygen and nitrogen atoms they produce the dazzling light shows that are the aurora.

Beautiful they may be but the invisible flows of particles and magnetism can damage electrical power grids and also affect satellites operating in space.

The lights can be in place day and night but are not bright enough to be visible in daylight. For the same reason in cities or towns with lots of light pollution you are unlikely to get good viewing.

Auroras tend to be more frequent and spectacular during high solar sunspot activity and these cycle over periods of approximately 11 years. That is what is happening now.

Some displays are particularly spectacular and make the headlines. This happened in August-September 1859, in February 1958, which I remember seeing as a London schoolboy, and in March 1989 the last time really good sightings were possible in southern England.

Last February produced spectacular solar activity and a few relatively clear nights again gave some lucky stargazers a chance to see the spectacular and colourful light show.

This year has been even better and there is a good chance that the shows aren’t over. Keep your eyes on those northern skies.

This article first appeared in the Morning Star 26 March 2015.

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