“The finding of Dr. Bossert’s paper is extremely important because it not only provides us with new means to study aurora, but also a deeper understanding of how the space environment and Earth’s upper atmosphere are intertwined,” says University of Alaska Fairbanks geophysicist Doğacan Su Öztürk, who was not involved in the new work.
Why the northern lights are showing up in unexpected places
The aurora borealis and aurora australis have dazzled us in the night sky for centuries. Here’s what causes them—and why we’re seeing them more lately.
By Kieran Mulvaney
Published May 17, 2023
• 6 min read
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No matter how many times you see them, the northern lights, or aurora borealis—and their Southern Hemisphere equivalent, the aurora australis—are an ethereal, breathtaking sight. Dancing silently in Earth’s upper atmosphere, they form iridescent sheets of green and red (or sometimes blue and purple) light.
Common in polar and sub-polar regions, auroras can sometimes be seen at lower latitudes, including recent dazzling displays as far south as Florida and England. If you live in the the contiguous United States, but it seems like you’ve been able to see auroras far more often than you normally would—well, you’re right. Here’s why.
What are auroras, or the northern and southern lights?
Italian astronomer Galileo Galilei, coined the term aurora in 1619 after the Roman goddess of dawn—mistakenly believing it to be the reflection of sunlight off the atmosphere.
In fact, both northern and southern lights are caused by the interaction of gases in Earth’s atmosphere with the solar wind: a stream of electrically charged particles, called ions, that shoot out from the sun in all directions.
When the solar wind reaches Earth, it slams into the planet’s magnetic field, producing currents of charged particles that flow toward the poles. Some of the ions become trapped in a layer of the atmosphere called the ionosphere, where they collide with gas atoms—primarily oxygen and nitrogen—and “excite” them with extra energy. This energy then gets released as particles of light, or photons.
Why are auroras green, red—and sometimes blue or purple?
An aurora’s colors signify where in the atmosphere, and with which gases, all of this is happening.
For example, it takes almost two minutes for an excited oxygen atom to emit a red photon, and if one atom collides with another during that time, the process may be interrupted or terminated. So, when we see red auroras, they are most likely at the highest levels of the ionosphere, approximately 150 miles (240 kilometers) high, where there are fewer oxygen atoms to interfere with one another.
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In contrast, green photons are discharged in less than a second, so are more common in moderately dense parts of the atmosphere, 60 to 150 miles (100 to 240 kilometers) above Earth’s surface.
In the very thick lower atmosphere, less than 60 miles (100 kilometers) above the planet’s surface, we see a purplish mixture of red and blue lights—the signature colors of molecular nitrogen.
The Shimmering Colors of the Aurora
If the magnificent spectacle of the Northern Lights could be summed up in one single word it would be Colors. The powerful light-show that graces the skies of the Far North indeed takes several forms and many colors, each one even more beautiful than its predecessors. Today we will be looking at the different shades of the luminescent Aurora.
To start with, most people would associate the Northern Lights with the color green. The color green is indeed by far the most common color to appear in Northern Lights pictures and this since the beginning of Aurora photography. However, the intensity of the green color is often not as powerful seen with naked eyes as when seen through a camera lens. Indeed, the bright, almost neon-green color of the pictures correspond most of the time to a somewhat paler hue appearing in the skies.
The Bright Green of the Aurora is something that mostly exists in the memory cards of digital cameras.
The green color that can be experienced in nature is indeed rarely as flashy as photographs could make one believe. There are two main reasons for that: 1) Most Northern Lights pictures are taken with a long-exposure shot, which can create much brighter and more saturated pictorial representation of an otherwise much darker scene. 2) The human eye is sadly not able to pick up colors very well in low-light conditions. Camera lens don’t discriminate their capture of color in such a manner and as a result night-time pictures of the Aurora almost systematically feature more saturated colors than what can be seen in real life. The green of the Aurora tends then to liken either Emerald or… Artichoke!
Colors of the Northern Lights are generally not as saturated when looked at with bare eyes.
But when the Aurora gets more powerful, the lower parts of the Auroral formation tends to become saturated with color. It might take a more yellowish/brownish color but this state is simply transitory as the next step is most often the birth of a thin stripe of purple under the greener top. This violet band most often appears as a pale plume of lavender and pearly purple shinning brightly during the strongest moments of an Auroral show. This beautiful light is however very fickle and is most of the time observed for less than twenty seconds at most so for once it is often easier to see it with naked eyes than taking a picture of it.
When the Aurora gets really strong, hues of purple can sometimes be seen at the bottom of the auroral structure
In some even rarer cases, the Aurora can develop a red or reddish upper-part. This colorful light, generally appearing on the top of band-shaped Northern Lights can quite frequently be photographed using a very long exposure time (generally close to thirty seconds). In these cases, this upper red coloring cannot really be seen with naked eye. However, when extremely powerful Auroras are unleashed, it might become visible for some time; still, as is the case for other colors, the red of the Northern Lights appears somewhat differently with the naked eye and on the camera. On pictures, we’re talking about bright, blood red, but with the naked eye the color is significantly darker and somewhat brownish, a bit like Carmine.
The red color on the upper part of the Northern Lights can sometimes be spotted but is overall extremely rare
All in all, the colors of the Northern Lights tend to vary a lot, even during a single sighting. The fact that our eyes cannot process the full extent of the Aurora (which happens, let’s not forget, at about 80 kilometers of altitude) mean that even simply focusing on the light can be extremely hard, which leave the gas-like shapes even more mysterious that they already are. In the end, in the occasion of powerful Solar storms, the beautiful Aurora Borealis can even become absolutely overwhelming with light and colors flying directly overhead. This rare and majestic phenomena, the Corona represents the pinnacle of the Aurora hunter’s search for the most beautiful light hidden in the Northern darkness…
The flickering of Light which is characteristic of the Aurora involved constantly shifting hues of lights
Pictures Sources:
- (1) A Bright-Green Aurora in Western Iceland. Ⓒ Lyonel Perabo (2014)
- (2) A dim-green Aurora near Tromsø. Ⓒ Lyonel Perabo (2013)
- (3) An Aurora with a purplish lower band. Ⓒ Lyonel Perabo (2015)
- (4) A Red-Green Aurora in Western Iceland. Ⓒ Lyonel Perabo (2014)
- (5) Picture of a Corona near Tromsø. Ⓒ The Northern Lights Blog (2015).
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Briley Lewis (she/her) is a freelance science writer and Ph.D. Candidate/NSF Fellow at the University of California, Los Angeles studying Astronomy & Astrophysics. Follow her on Twitter @briles_34 or visit her website www.briley-lewis.com.
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