Photo: Barry Becker Video: Barry Becker
“I dazzle the auroras with a sheet of silk that changes color and intensity as they are blown by a gentle breeze across the starry black sky.”
Barend (Barry) Becker – Senior Meteorologist, Casey Research Station, 2022.
The aurora australis or ‘southern lights’ are the brilliant curtains of green, red and sometimes violet light that appear in the night sky around the south magnetic pole. In the Northern Hemisphere they are called aurora borealis or ‘northern lights’.
At Australia’s Antarctic and sub-Antarctic stations, expeditioners are among the lucky few who regularly witness light shows during the long winter darkness (see their aurora photography tips at the end of this feature ).
But auroras are more than pretty lights.
Aurora over Casey Bay. Photo: Justin Chambers Green aurora over Mawson station sign. Photo: Brian Jury
Auroras are a form of space weather that occurs between 90 and 250 kilometers above the ground, and they provide scientists with important clues about what’s happening at the edge of space.
Australian Antarctic Program scientists study a range of atmospheric phenomena that also occur in the auroral zone and, below it, in the stratosphere (12-50 km) and mesosphere (50-80 km).
His research is contributing to improved weather and climate prediction models, and to a better understanding of the complex atmospheric processes that link what happens in Antarctica to the rest of the world (more information at Global Atmospheric Links).
Auroras 101
So how do auroras form? Let’s start with the basics.
Auroras occur in Earth’s upper atmosphere, above about 90 kilometers, when oxygen atoms (O) and nitrogen molecules (N2) interact with a stream of charged particles emitted by the sun.
This “solar wind” comes out of the sun and connects with the Earth’s magnetic field at speeds of 400 to 700 kilometers per second.
While most of the charged particles in the solar wind blast off into space, some of them get trapped within the Earth’s magnetic field and are accelerated toward the magnetic poles at tens of thousands of kilometers per second.
An auroral oval over Antarctica. (Photo: NASA)
When these particles collide with gases in the Earth’s atmosphere, the energy from the collision can be released as light. Depending on the altitude, the gas (O or N2), and the energy of the collision, the light is green, red, or violet.
Although some auroras may appear pink or white, this is usually due to poor light perception or camera color bias.
Here’s how to make an aurora…
Video: NASA
This NASA video shows an aurora over Antarctica as seen from the International Space Station.
Although auroras can occur at any time, we can only see them in the dark and usually within the auroral oval, a belt around the magnetic poles that expands or contracts depending on the strength of the solar wind
In the Southern Hemisphere, the auroral oval is generally found over the Southern Ocean and parts of Antarctica. During intense solar activity, it can extend as far north as Tasmania and the southern part of mainland Australia.
In the Northern Hemisphere, the aurora can be seen in places like Greenland, Scandinavia, and parts of Canada and Siberia.
Recipe for an aurora
When you look at what it takes to create an aurora, it’s amazing that the phenomenon occurs. Consider these ingredients:
- A solar wind of charged particles flowing from the sun
- A planet with a magnetic field
- A way to accelerate particles, or “magnetic reconnection”
- A planetary atmosphere containing oxygen and nitrogen
- The ability of these gases to emit light in the visible spectrum (that is, what we can see)
- perfect weather
And if you want to see an aurora, you have to be in the right place.
Between 2019 and 2021, Davis Station expeditioner Will Kenton was in the right place at the right time to capture this incredible series of aurora…
solar wind
Let’s look at the first ingredient needed to create an aurora: the sun.
This star at the center of our solar system is a glowing ball of plasma made up of hydrogen and helium gas, heated to 15 million degrees Celsius by nuclear fusion reactions in its core.
Video: NASA Goddard Space Flight Center
The sun loses about 4.29 million tons of its mass per second. About 70% of this mass is released as energy (heat and light) and 30% is ejected as a stream of charged particles called the solar wind.
The solar wind consists mainly of negatively charged electrons and positively charged protons, moving at incredible speeds.
To escape the sun’s gravity, they must be moving at a speed of at least 618 kilometers per second! But even at these speeds, it takes two to four days to cover the 150 million kilometers distance between the sun and the earth.
The intensity of the solar wind varies depending on solar activity during an 11-year solar cycle. Solar activity is driven by changes in the sun’s magnetic field during this cycle, and includes sunspots, coronal mass ejections, solar flares, and other phenomena.
In fact, we are currently in an active solar cycle, with a maximum expected in July 2025.
So what happens when the solar wind hits the earth?
The Earth as a magnet
The second ingredient in the aurora recipe is the Earth’s magnetic field.
The earth has a solid inner core and a molten outer core consisting mainly of iron and nickel. The intense heat of the core, between 5000°C and 7000°C, creates movement within the liquid outer core, which generates electric currents and turns the earth into a magnet.
The resulting magnetic field, or “magnetosphere,” extends from Earth’s interior to outer space. Like a giant shield, it protects the earth from the sun’s solar wind, which would otherwise strip the delicate atmosphere that protects us from ultraviolet (UV) radiation.
The Earth’s magnetic field forms closed semicircular loops around the planet, radiating from the north and south magnetic poles (near the geographic poles).
When the solar wind hits Earth’s sunward magnetic field, it compresses and disrupts these field lines, creating a “bow wave” ahead and a long tail on Earth’s nightside.
This disruption causes a temporary distortion in the shield, allowing some electrons to break through the magnetosphere and become trapped inside.
When the shield realigns (a process called magnetic reconnection, the third ingredient in this recipe), it provides a massive energy boost to the electrons, which are driven even faster by the field lines toward the poles, often between 18,000 and 38,000 kilometers per second. .
These runaway solar wind particles can cause space weather that interferes with satellites, spacecraft, aircraft, telecommunications and power supplies. They also create auroras.
Green aurora over Casey. Photo: Barry Becker
Southern light show
When electrons spiraling down magnetic field lines near the poles collide with atmospheric gases, particularly oxygen atoms (O) and nitrogen molecules (N2), between 90 and 250 kilometers above the land
The collisions transfer energy from the solar wind electrons to the O and N2 gases, exciting them to a higher energy level. However, these excited gases can only retain this extra energy for a short time before losing it, in two ways:
- collide with other gases in the atmosphere: the lower the altitude, the denser the atmosphere and the more likely the gases are to collide with each other.
- emitting the energy as a photon of light, either red, green or violet, to create an aurora.
Bands of aurora over the googie huts on Béchervaise Island. Photo: Peter Layt
Oxygen atoms emit red light at high altitudes, between 150 and 250 kilometers, when they collide with electrons traveling at least 830 kilometers per second.
Oxygen holds this collision energy for about 120 seconds before it is released as light. Red emissions appear only in the thin upper atmosphere, where excited oxygen atoms are less likely to…