Different firms around the globe spend millions of dollars to put together spectacular light shows. But surpassing the beauty of all these human-made light shows are the ones occurring magnificently on their own, absolutely free of cost, dancing on Earth’s North and South poles: The Aurorae.

This image of a colourful aurora taken in Delta Junction, Alaska, on 10 April 2015.
 Image courtesy: Sebastian Saarloos.
This image of a colourful aurora taken in Delta Junction, Alaska, on 10 April 2015.
Image courtesy: Sebastian Saarloos.

What is an “Aurora”?

What is an aurora? The term aurorae (plural for aurora) refers to the natural display of light in Earth’s sky that mostly occurs in high-latitude regions. The aurorae found at the North and South pole are called the Northern and Southern Lights (also known as Aurora Borealis and Aurora Australis, respectively). The Aurora Borealis is one of the world’s seven natural wonders that people dream of witnessing once in a lifetime. However, strong solar storms sometimes change the course of auroras, thereby shifting them towards the equator as well (like during the Carrington event).

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The science behind the origin of aurorae

The Sun is responsible for the existence of the solar system and our presence on Earth. However, it showers us with warmth and energy and bundles of charged particles every second. Our Earth’s protective magnetic field shields us from most of this unwanted energy and particles, and that’s why we don’t even notice them.

Most of the highly energetic and charged particles escape from the sunspot regions on the surface of the Sun. The sunspots burp out particles of plasma, known as the solar wind, deep into space. These winds take around 40 hours to reach Earth and carry a stream of charged particles like electrons accelerated to extremely high speeds.

When these highly energetic particles reach Earth from the Sun, they enter the Earth’s magnetic field and travel along these lines until they reach the North or South pole, where they enter the Earth’s atmosphere. After entering the atmosphere, the charged particles interact with the atoms and molecules present and eventually excite them.

When the excited atoms and molecules get de-excited, and their electrons make a transit to lower energy levels, it leads to an emission of photons of a specific wavelength which further appear as lights dancing in the sky.

The formation of Aurorae (Image : Earthsky.org)
The formation of Aurorae (Image : Earthsky.org)

The colors of aurorae

The color of an aurora light is decided by the atoms/molecules emitting the photons and the energy difference between the excited and stable configuration of the atom under consideration. Moreover, the altitude of de-excitation also affects the color of aurorae. Mostly, the light emitted from oxygen atoms is either red (at a wavelength of 630 nm) or green (at 558 nm), depending upon the energy levels the ‘excited’ electron dropped between. The blue and red colored aurorae are the most common ones. However, other colors also appear on rare occasions catering to the mixing of light, the energy of charged particles, and other atoms.

Aurora australis as seen from from the ISS in 2017
Aurora australis as seen from from the ISS in 2017

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Non-terrestrial aurorae

An aurora can form on any planet or moon,  provided it has an atmosphere. So, yes! Aurorae are not only visible on Earth but are also prevalent at some other places in our solar system. For instance, both Jupiter and Saturn have magnetic fields that are stronger than Earth’s and Auroras have been observed on both these gas giants.

While the aurora on Saturn seems to be powered by the solar wind, Jupiter’s aurorae are associated with the plasma produced by its volcanic moon Io in a very complex manner. Even Venus, Mars, Uranus, and Neptune have been observed as homes to these scenic lights.

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An aurora captured on Jupiter’s north pole ( Image: NASA/ESA)

In July 2015, the first-ever extra-solar aurorae were discovered over the brown dwarf star LSR J1835+3259. These aurorae mainly appeared to be of a reddish hue and were a million times brighter than the northern lights. It is speculated that such a bright light resulted from the charged particles interacting with hydrogen in the star’s atmosphere. However, a clear explanation is yet to come.

Learn Astrophysics at Home

Did you always want to learn how the universe works? Read our 30-article Basics of Astrophysics series absolutely free of cost. From the popular topics such as stars, galaxies, and black holes to the detailed concepts of the subject like the concept of magnitude, the Hertzsprung Russell diagram, redshift, etc., there is something for everyone in this series. All the articles are given here. Happy reading!

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