Aurora Borealis – The play of colours over St Andrews

During the night of 27 February, 2014 a rare phenomenon took place on the sky of St Andrews. Around 10 pm the lucky ones saw the amazing red and green splendour of the Aurora.

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Red and green lights of the Aurora Borealis with the Castle of St Andrews in the front. (Credits go for Pasquale Galianni, astronomer of the University of St Andrews)

Red and green lights of the Aurora Borealis with the Castle of St Andrews in the front. (Credit: Pasquale Galianni)

Everyone heard of the bright, dancing, colourful lights of the Aurora Borealis. Some have seen it by their own eyes, others have seen breathtaking pictures of it. But what is the Aurora exactly?

The Northern Lights are natural light phenomena, which occur at high latitudes on both hemispheres of the Earth (called Aurora Borealis on the North and Aurora Australis on the South). It is the result of the collisions of charged particles coming from the Sun as solar wind and the upper part of a planet’s atmosphere.

In the upper corona of the Sun the velocity of the thermal motion of the particles become higher than the escape velocity. This results in a continuous material loss from the Sun in the form of solar wind. These charged particles (ions and electrons) hit the Earth’s magnetosphere and tie to it. The magnetosphere accelerates some of these particles towards the Earth’s surface. As they reach the upper atmosphere they collide with atoms and molecules releasing kinetic energy, which we is seen as the lights of the Aurorae. The more active the Sun (which means more solar wind) the more frequent the Aurorae.

The colour of the lights depends on the atom/molecule the energetic particle collides with. The most commonly seen type is the green Aurora. At mid altitudes (~ 100 km), where the concentration of oxygen atoms is fairly high, the collisions between atoms and ions/electrons releases energy at ~560 nm, which is in the green part of the spectrum. At the highest altitudes (up to ~300 km) the oxygen atoms emit around 630 nm (red part of the spectrum). Because of the lower concentration of the atoms in this part of the atmosphere, red Aurorae are seen very rarely and only when the Sun is around its activity maximum. The blue colour is the result of the collision with molecular nitrogen. This takes place at lower altitudes, where the amount of atomic oxygen is reduced.

Aurora Borealis seen from the Observatory of St Andrews. (Credit: Diana Juncher)

Aurora Borealis seen from the Observatory of St Andrews. (Credit: Diana Juncher)

The Aurora is not unique on Earth in the Solar System. Planets like Jupiter or Saturn, which have stronger magnetic fields than Earth exhibit even more spectacular light phenomena. Auroral light was observed on Uranus and Neptune as well.

The Northern Light we saw last Thursday was the result of a very energetic solar flare which was erupted on the 25 Feb (00:25 UTC). NASA’s Solar Dynamics Observatory (SDO) captured the gigantic flare in different wavelengths. The one seen below is a composite image of two wavelengths of extreme ultraviolet light (171 and 304 Angstroms). The flare is classified as X4.9 which means it is one of the most powerful types. As the Coronal Mass Ejection (CME) originated from this flare reached the Earth’s magnetosphere the beautiful dance of lights appeared on our sky.

Solar flare erupted on at 00:25 (UTC) 25 Feb as capured by the SDO. This image is the combination of two wavelengths of extreme ultraviolet light (171 and 304 Angstroms). (Credit: NASA/SDO)

Solar flare erupted on at 00:25 (UTC) 25 Feb as capured by the SDO. This image is the combination of two wavelengths of extreme ultraviolet light (171 and 304 Angstroms). (Credit: NASA/SDO)

And to finish with, here is a nice GIF made from some of Diana’s photos. Thanks to Inna Bozhinova for creating this short “movie” for us!

Aurora Borealis on the 27 Feb. (Credit: Diana Juncher, Inna Bozhinova)

Aurora Borealis on the 27 Feb. Click on the image to see it in a better quality. (Credit: Diana Juncher, Inna Bozhinova)

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