Courtesy of Unsplash
Photo by Kari Saari
Dunes auroras can spread horizontally for hundreds of miles.
Plus, an enlightening guide to the other types of auroras you may see.
Big news for aurora chasers—there’s a new, elusive pattern to keep a watch for next time you’re viewing the celestial phenomenon. Up until recently, scientists recognized five distinct types of Northern Lights. But thanks to avid aurora enthusiasts and amateur photographers, we’re now learning that there’s a sixth shape—and it’s really impressive.
Usually when you see the Northern Lights, they’re arranged like a vertical curtain in the sky. But on October 17, 2018, skywatchers in Finland and Sweden noticed that the Northern Lights were spreading out horizontally, undulating like a wave and stretching toward the equator. The sight led to new research as well as the revelation that this style of aurora has been spotted in other places as well, including the United States and Canada. Last Tuesday, January 28, 2020, an article in the journal AGU Advances officially presented this pattern as a new type of aurora, now known as “dunes” because it looks like wind-sculpted ridges of sand.
(Timelapse by Kari Saari)
If this is ringing a bell, you may be remembering the 2018 discovery of STEVE (Strong Thermal Emission Velocity Enhancement), a thin, vertical, aurora-like band of purple that initially baffled scientists. The phenomenon had actually been known to photographers for decades, but it gained attention in 2018 when the scientific community began to investigate its origin. It turns out, STEVE is formed in a different atmospheric level and in a different way than auroras are.
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Auroras are created in the thermosphere and are formed when solar wind—or a stream of charged particles emitted from the sun—hits the Earth’s magnetic field and excites gases like nitrogen and oxygen in our atmosphere. The different colors—pink, green, yellow, blue, violet, and occasionally orange and white—are released when the resulting energy starts to wear off. When people talk about the right conditions for the Northern Lights, they aren’t just talking about clear skies—good auroral conditions occur when there’s also enough activity on the sun to form solar wind.
The new aurora shape looks like a wave and can stretch out hundreds of miles towards the horizon. We’re still learning about the conditions under which dunes are formed, so we don’t yet know when or where they’re most likely to occur.
Arcs resemble a rainbow, a curve from one side of the horizon to the other. They are the most common type of aurora and usually occur when aurora conditions are quiet or growing in intensity. It’s the pattern you’ll usually see in lower latitudes; in higher latitudes, you might be able to see that it breaks in two on the right-hand side. If you’re lucky enough to stand under an arc, it would look like a thick river of light.
These ribbon-like aurora formations are the ones you are probably hoping to see. They appear when auroral conditions are more active, so arcs can turn into bands. When things get really intense, they may look like they’re dancing.
Also called beams or rays, pillars can be seen alone as a streak or two in the sky or a cluster of streaks. But they can also be a feature of an active arc or band aurora. These are the vertical bars of light that seem to reach upwards. They can sometimes be as tall as 375 miles from the green base and purple canopy.
Similar to pillars, diffuse is both a shape of aurora and a way to describe other types. Enthusiasts will describe arcs or bands as being more diffuse if you can’t see defined pillars making up the pattern. But auroras can also present as light glow known as diffuse, which has no discernible shapes or features. It’s rare, and you usually need special equipment to see it.
The corona is the most impressive shape and the one most sought after by aurora chasers (at least until the discovery of dunes). The crazy, colorful vortex effect appears when you are standing directly under a display of pillars—when it happens, the base of the aurora is only about 50 to 60 miles above your head. People have described coronas as looking like angels or butterflies or dragons—a stunning photo of a “dragon aurora” by Jingyi Zhang was NASA’s Astronomy Photo of the Day on February 18, 2019. These are rare—they happen sporadically at high latitudes during periods of intense solar activities, but you would need a major solar event to see one at lower latitudes.
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