There may be no phenomena more magical on Earth than the aurora borealis and the aurora australis: the northern and southern lights. Although we know some of the science behind them, once you’ve witnessed them for yourself, you feel in your heart that they are pure magic; that they come to you from a fairy-tale place.
Having such emotional power over us, it’s no wonder that the auroras inspire countless legends. In North America, some people believed that the lights were the spirits of the dead playing football with the head of a walrus. Others thought the aurora was a narrow, torch-lit path to guide departed souls to heaven. Around the 16th century, people in Europe treated red auroras as frightening omens. It’s clear that the lights hold a special place in every culture and each era of history.
Auroras are visible nearly every night near the Arctic and Antarctic Circles, which are about 66.5 degrees north and south of the equator. If you could look at Earth from space, you would see a ring-shaped aurora spanning about 2,500 miles around both poles. This auroral zone covers central and northern Alaska and Canada, Greenland, Russia and northern Scandinavia in the Northern Hemisphere, and Antarctica in the Southern Hemisphere.
But a long time ago, some people in Tasmania got extremely lucky regarding the southern lights. They experienced what’s known as the Laschamp Geomagnetic Excursion: where the Earth’s magnetic field “switched,” and the skies lit up like fireworks.
Shaping the kaleidoscopic shades
The activity that creates auroras begins on the sun. A ball of superhot gases, the sun is made up of electrically charged particles called ions. The ions, which continuously stream from the sun’s surface, are called the solar wind.
As the solar wind approaches the Earth, it meets the Earth’s magnetic field. Without this magnetic field protecting the planet, the solar wind would blow away the Earth’s fragile atmosphere, which would prevent all the life that we now know. Fortunately, most of the solar wind is blocked by the magnetosphere, forcing the ions to go around the planet and thus continue to travel farther into the solar system.
Although most of the solar wind is blocked by the magnetosphere, some of the ions become briefly trapped in ring-shaped holding areas around the planet. These areas, in a region of the atmosphere called the ionosphere, are centered around the Earth’s geomagnetic poles. The geomagnetic poles mark the tilted axis of the Earth’s magnetic field. They lie about 800 miles from the geographic poles, but they are slowly moving.
In the ionosphere, the ions of the solar wind collide with atoms of nitrogen and oxygen from the Earth’s atmosphere. The energy released during these collisions causes a colorful glowing halo around the poles: an aurora. Most auroras happen about 60 to 620 miles above the Earth’s surface.
The most active auroras occur when the solar wind is the strongest. The solar wind is usually constant, but solar weather—the heating and cooling of different parts of the sun—can change daily.
Solar weather is often measured in sunspots. Sunspots are the coldest parts of the sun, and they appear as dark blobs on its white-hot surface. Solar flares and coronal mass ejections—sudden, extra bursts of energy in the solar wind—are associated with sunspots. Sunspot activity is tracked over an 11-year cycle. Bright, consistent auroras are most visible during the height of sunspot activity.
Some increased activity in the solar wind happens during every equinox. These regular fluctuations are known as magnetic storms. Magnetic storms can lead to auroras being seen in the midlatitudes during the spring and autumnal equinoxes.
The colors of the aurora vary, depending on altitude and the kind of atoms involved. If ions strike oxygen atoms high in the atmosphere, the interaction produces a red glow. This is an unusual aurora; the most familiar display, a green-yellow hue, occurs as ions strike oxygen at lower altitudes. Reddish and bluish light that often appears in the lower fringes of auroras is caused by ions striking atoms of nitrogen. Ions striking helium and hydrogen atoms can produce blue and purple auroras, although our eyes can rarely detect this part of the electromagnetic spectrum.
Creating the colorful chaos
In 2014, Dr. Agathe Lise-Provonost, a McKenzie Fellow from the School of Earth Sciences at the University of Melbourne, Australia, traveled to a small subalpine lake, Lake Selina, in western Tasmania. She was accompanied by a research team that helped her construct a makeshift floating platform rigged to two inflatable rafts that could be used to drill down into the sediment.
The scientists managed to extract a 270,000-year-old core of sediment that contained a climate, vegetation and paleomagnetic record of the area. Magnetic particles that erode from rocks make their way to a lake by wind or water, and then they settle down to the lake’s bottom. Magnetic particles act like tiny compass needles, aligning with the Earth’s magnetic field. As these particles accumulate and become buried, they are locked in place, leaving a history of the planet’s magnetic field. The deeper the researchers drilled, therefore, the further back in time they went.
First, the team looked to accurately date the core’s layers. They found evidence of ecosystem changes that occurred as Tasmanian Aborigines arrived 43,000 years ago and managed the land over thousands of years. Abrupt changes that happened since the arrival of Europeans 200 years ago were also documented.
Publishing their results in the journal Quaternary Geochronology in March 2021, the scientists stated that the sediment core showed that 41,000 years ago, people in Tasmania would have seen spectacular auroras when the Earth’s magnetic field flipped; and for a few thousand years, north was south, and south was north. The strength of the Earth’s magnetic field almost vanished; and because of this loss of the planet’s “shield,” there was a big increase in cosmic and solar particles bombarding the Earth.
If such an event, known as a “geomagnetic excursion,” were to happen today, satellites would be rendered useless, smartphone navigation apps would fail and there would be major disruptions of power distribution systems.
It’s hoped that this research will lead the way for more studies of past geomagnetic field behavior—using Australian lakes and other geological materials such as cave deposits, lava flows and fired archaeological artifacts—for developing new paleomagnetic dating tools and for improving models of the Earth’s magnetic field to, perhaps one day, predict the next geomagnetic excursion.
Making midair magic
Adding to their enchantment, auroras not only happen on Earth—other planets have them, too. If a planet has an atmosphere and a magnetic field, it probably has auroras. In fact, stunning auroras have been detected on Jupiter and Saturn.
Luckily, on our home planet, auroras happen 24 hours a day, seven days a week, 365 days a year. Seeing them with your own eyes, though, is still a holy grail for many astronomy lovers and nature travelers, alike. Like most magical things, the northern and southern lights require that you be in the right place at the right time.
The ancient Tasmanians sure were.
Here’s to finding your true places and natural habitats,