The aurora – a natural light-show

From certain parts of the world, the night sky is frequently lit up by a phenomenon known as the aurora. This sometimes spectacular light-show occurs in our upper atmosphere, but it is triggered by events on the Sun.

A bright aurora viewed from Vogar, on Iceland’s Reykjanes peninsula, on October 5th/6th, 2018. Image credit: Paul Sutherland

This display can range from a gentle glow to spectacular curtains of light dancing rapidly against the starry background. It occurs at high latitudes around the Earth’s magnetic poles.

When it occurs in the northern hemisphere, it is known as the aurora borealis, which means northern dawn, and in the south as the aurora australis or southern dawn. They are commonly known today as the northern or southern lights.

The magnetic poles do not coincide with the geographical poles on the Earth’s axis, but are sufficiently close that the aurora is mainly seen around Arctic and Antarctic regions.

The appearance of a bright aurora was regarded with awe by ancient peoples. They still make for an awesome sight, but the science behind them is better understood.

Related: How to see the aurora on a budget

Stone age cave paintings in Europe dating back 30,000 years are believed to depict early displays. The Greek philosopher Aristotle compared it to fires on Earth in 344 BC. Different civilisations believed they were seeing the warring spirits, celestial caves, dancing gods or a vision of the afterlife.

The most powerful displays penetrate further from the polar regions, and rare major shows have been seen from southern parts of Europe or the United States. In 34 AD, Roman troops marched on the city of Ostia in Italy because a bright aurora convinced Tiberius Caesar that the town was in flames.

The aurora borealis was given its name by Galileo who mistakenly thought it was caused by sunlight reflected from the atmosphere. In the 18th century, the leading American thinker Benjamin Franklin suggested that the aurora was created by electrical charges concentrated around the poles and intensified by snow and moisture.

A key event in our understanding of the aurora came in September 1859, when a brilliant red, green and purple light show encircled the Earth, and was visible even in the tropics.

This rare and dramatic display happened just a day after a British astronomer, Richard Carrington, had observed two blinding beads of white light appear briefly on the Sun above a sunspot. It was the earliest recorded solar flare. Astronomers soon realised that the flare on the Sun and the incredible aurora must be linked.

What causes the aurora?

Today we know that the aurora is produced when a high-speed stream of charged particles from the Sun forming the solar wind collides with a protective bubble around the Earth called the magnetosphere. Electrons react with oxygen and nitrogen atoms in the higher levels of the atmosphere, making them glow. The effect is similar to switching on a fluorescent strip light.

A diagram, not to scale, shows how the solar wind impacts the Earth’s protective magnetic field. Image credit: Steele Hill, NASA/ESA

The effects from the solar wind are known as space weather. Monitoring of the Sun by early space observatories revealed that powerful bursts of solar wind are associated with massive eruptions of energy called coronal mass ejections, or CMEs. These massive bursts can hurl vast quantities of electrically charged plasma and radiation out from the Sun. If a CME occurs on part of the Sun facing Earth, then the stage is set for phenomena such as the aurora.

But as well as producing beautiful celestial light shows, major ejections aimed in our direction pose a threat to our modern infrastructure, including power grids, satellite electronics and even the lives of astronauts in orbit. Flares and CMEs are far more frequent during the active phase of the Sun’s cycle.

Streams of particles are also sent flowing into space from coronal holes, cooler regions of the Sun’s atmosphere where its magnetic fields open to release the solar wind.

Our Earth’s magnetosphere normally shields us from deadly radiation from space and blocks out most of the solar wind. Our natural magnetic field is thought to be generated by a dynamo process, produced by the movement of liquid within our planet’s core. But like a bar magnet, field lines in the magnetosphere channel some of this stream from the Sun towards the magnetic poles and electrons collide with atoms and molecules in the ionosphere, releasing energy that causes them to glow brightly, between 100 and 1,000 km (60 – 600 miles) above the Earth.

The aurora forms in the shape of ovals around each of the north and south magnetic poles. You can see the position and current level of activity of these ovals by visiting websites including the Space weather Prediction Center operated by the National Oceanic and Atmospheric Administration. The ovals expand during geomagnetic storms.

As already noted, aurorae are mostly seen in polar regions such as Alaska, the Yukon, Iceland and northern Scandinavia. but more powerful bursts of particles from solar storms produce the displays that are seen at latitudes further from the poles.

A diagram demonstrates the field lines in the Earth’s magnetic field which channel electrons in the solar wind towards the magnetic poles. Image credit: Peter Reid/University of Edinburgh

The effects of the solar wind and the Sun’s magnetic field can be felt throughout the Solar System, far beyond the planets, and this region of influence is known as the heliosphere after Helios, the Greek sun god.

What does the aurora look like?

The aurora can take various forms from a gentle glow near the horizon to arcs, rippling bands and streaks of light. Brighter and more dramatic shows can resemble constantly shifting curtains or even a corona where rays radiating from overhead appear to fill the sky.

The familiar asterism known as the Plough or Big Dipper shines through an aurora over Vogar, Iceland, on October 5th/6th, 2018. Image credit: Paul Sutherland

The most common colour for an aurora is green but other displays can be red or purple depending on which atoms of gas the electrons from the solar wind have struck. Though these colours appear particularly striking in photographs, this is often due to the time exposures used and the brightness and colour usually appears more subtle to the eye.

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