Aurora: The Biggest Lightshow on Earth
Aurora over Alaska. Credit: Sebastian Saarloos via NASA

Aurora: The Biggest Lightshow on Earth

19/09/2018Written by Toby Raine

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Mysterious lightshow

The northern lights are one of the most awe-inspiring displays of natural beauty that this planet has to offer.

Since ancient times, people have looked to the sky and observed the dance of these flame-like columns of light, often interpreting meaning into their mysterious appearance.

As we’ve gained more of an understanding of the northern lights as well as their counterparts in the south (the southern lights), we now know exactly what is needed to create them, both here on Earth and on some of the biggest planets in our Solar System.

What are the northern lights?

What are the northern lights?
Aurora seen from the International Space Station. Credit: NASA

The northern and southern lights are examples of what we call ‘aurora’, which is the Latin word for dawn. Aurorae were once thought to be the first light of dawn. Italian astronomer Galileo Galilei combined this name with the Greek term for the northern wind to give the alternate name for the northern lights – Aurora Borealis. They most commonly appear in the night skies of very northern areas (i.e. high latitude areas) as a mixture of green, blue, and red lights that seem to flow like a curtain in the breeze.

How do they form?

How do they form?
Diagram showing the formation of aurora. Credit: NOAA
How do they form?
Aurora over Alaska. Credit: Sebastian Saarloos via NASA

There are three key ingredients to create an aurora: a flow of high-energy charged particles, a magnetic field, and an atmosphere of gas.

In our Solar System, the charged particles come from our Sun’s solar wind. The solar wind is a constant stream of charged particles sent out in all directions from the Sun’s upper atmosphere (the corona). These charged particles hurtle towards us at tremendous speeds, but luckily, due to the movement of molten iron underneath the Earth’s rocky crust, there is a magnetic field that protects our atmosphere.

The magnetic field acts like a big shield against the solar wind, deflecting away most of the particles from the Earth. However some manage to get through, following the donut shape of the Earth’s magnetic field towards the poles. As they do this, the particles bump into gases in the atmosphere and give these gas atoms a big kick in energy. This excess energy is given off as light, forming a bright oval of colour in the sky around the northern and southern poles of the magnetic field – the aurorae!

What causes the colours?

What causes the colours?
Aurora over the Pacific Northwest. Credit: ESA/NASA
What causes the colours?

So, what causes the different colours of the aurora? This is down to the types of gases in our atmosphere and how high they are above the ground. Different elements have their own way of emitting light when they absorb a lot of energy, like a unique signature. Each element will only emit certain very specific frequencies of light, which correspond to certain colours.

Just as rain drops can act like a prism and spread out the light from the Sun to make a rainbow, we can use tools called spectrometers to spread out the light from these gases. This allows us to compare the light we see to the known signatures of common gases such as oxygen, nitrogen, and hydrogen.

The iconic green of the aurora here on Earth is mainly due to atmospheric oxygen, which rapidly reduces in concentration below around 100 kilometres, resulting in abrupt ends at the lower edges of the aurora curtains.

Extraterrestrial aurora

Extraterrestrial aurora
Jupiter’s northern aurora, with Moon footprints. Credit: NASA/ESA/J. Nichols (University of Leicester)
Extraterrestrial aurora
Aurora on Saturn. Credit: NASA

Did you know that the Earth’s aurora isn’t the only show in town? We’ve also found aurora around other planets in the Solar System including Mars, Venus, and the gas giants.

Jupiter and Saturn both have very strong magnetic fields and extensive radiation belts resulting in some very active aurora. Unfortunately, we need some assistance to see their aurora as most of the light lies in the ultraviolet part of the spectrum, invisible to the human eye.

Researchers in the Radio and Space Plasma Physics group at the University of Leicester have produced some stunning images of the aurora around Jupiter and Saturn and have investigated the origin of these extraterrestrial aurora.

They have even seen effects from the moons of the gas giants. Moons such as Io and Europa regularly eject material into space, which then hits the magnetic field of Jupiter and leaves a visible ‘footprint’ in its aurora.

So, if you’re looking to the skies and you manage to catch a glimpse of the aurora, take a moment to appreciate this stunning interaction between the Sun and the Earth.

A beautiful reminder of the invisible shield that protects our atmosphere from the extremes of the cosmos.

About the author: Toby Raine is a physics student at the University of Leicester and works as a Science Interpreter at the National Space Centre.