A panorama photo taken in March 2017 in southern Alberta shows a green aurora sandwiched between two pinkish arcs known as the STEVE phenomenon. (Alan Dyer/VWPics/AP)

How to find STEVE, the purple streak that looks like an aurora but isn’t

Scientists say there could be good opportunities to spot STEVE in the night sky as solar activity increases over the next few years

9 min

She’s been chasing the northern lights for nearly a decade, so amateur photographer Donna Lach can easily recognize the green, red and purple curtains of light of the aurora borealis dancing across the sky. But one Sunday night in September 2015, while watching a rather lackluster aurora, she saw something she didn’t recognize. A strange mauve light accompanied by “a green blob” appeared to the west of the aurora.

“It was this pink arc, and at some time, it extended above me,” said Lach, based in the Canadian community of Plumas, Manitoba. “I didn’t really know what it was.”

She was seeing something rarer than an aurora. It was STEVE.

STEVE is not an aurora, but you can think of it as a shy, distant cousin. It looks as if it could be part of the family, but it has its own distinct style. The phenomenon generally appears as a long, slender purple-and-white arc, sometimes accompanied by a structure that looks like a green picket fence. It is fainter and narrower, and occurs at lower latitudes than most auroras. It is also harder to predict. (Then there’s the name, which we’ll address shortly.)

Seeing STEVE can be a matter of space physics serendipity. Lach has managed to photograph STEVE more than 20 times since that first sighting in 2015 — potentially the most of any individual on record — and has probably seen the ribbon of light even more. Many leading researchers in the field haven’t even seen the light phenomenon once.

Citizen scientists like Lach, in addition to satellite data and sky cameras, have been vital sources for researchers investigating STEVE — when it forms and how to recognize it. One citizen-science project, called Aurorasaurus, allows the public to report sightings and connects amateur photographers with scientists. Since STEVE’s formal identification in 2018, researchers, photographers and citizen scientists have learned what makes STEVE special. (A note: In a previous job at NASA, this reporter worked part time for Aurorasaurus and wrote news releases related to the discovery, though she did not previously work with any researchers mentioned in this piece.)

Solar activity is expected to be on the rise over the next few years, researchers say, so there could be good opportunities for the public to spot STEVE. During a severe geomagnetic storm last week, a few people photographed the phenomenon in the U.S. Such storms headed towards Earth can be difficult to detect with more than a day or two’s notice, but aurora and STEVE chasers can check the Space Weather Prediction Center for potential promising geomagnetic activity.

“Someone who doesn’t have a degree in it can still contribute to the scientific study,” said Lach, who recently retired as a school administrative assistant and now manages a farm.

Why is it called STEVE?

Naming a streak of light “Steve” may seem a bit random, and it is.

After her initial sighting in 2015, Lach along with other aurora chasers posted about the strange mauve arc online and even casually discussed it in person. They thought it might be a known phenomenon called a proton aurora, a type of aurora that is broad and diffuse and invisible to the naked eye, requiring instruments like a camera to see it. But scientists refuted that identification because this phenomenon was visually bright, narrow and structured.

In an effort to call it something besides a purple-y ribbon-y thing, Chris Ratzlaff, an aurora chaser and photographer around Calgary, Alberta, suggested something a little different: “Steve.” He pulled the name from the animated children’s film “Over the Hedge,” which he had recently watched with his kids. In one scene, the animal characters are frightened by an unfamiliar trimmed bush and decide to call it Steve.

As scientists began looking at satellite data and photos in more detail, they determined that this thin ribbon of light was actually a very fast stream of particles in the upper atmosphere that experience so much heat that they glow. A team of space physicists and citizen scientists published the initial study identifying Steve in 2018.

The scientists gave the phenomenon a backronym: Strong Thermal Emission Velocity Enhancement.

What are some impacts of STEVE?

Even if you haven’t seen the awe-inspiring STEVE, its impacts could be experienced by Earthlings in other ways.

STEVE researcher Toshi Nishimura recounted at least one instance in which a radio signal from a radar network disappeared for 30 minutes as STEVE appeared, but then returned once the ghostly light left the area, suggesting that the phenomenon can disrupt such signals. Similar blackouts can occur with space weather associated with auroras, but he said STEVE occurs in a different region. He said it "would have a larger impact on densely populated regions at lower latitudes than aurora.”

STEVE is important because Earth’s magnetic field “is doing something different from usual. … We still don’t understand why,” said Nishimura, who recently published a study on the mysteries of STEVE. He has yet to see the phenomenon in person.

How do you know if what you’re seeing is STEVE?

STEVE has gained popularity in the past decade, but potential observations date back to the 18th century. Of course, it wasn’t known as STEVE back then. Photographs were also unavailable early on, so sightings were sometimes described through text or sketches.

In a study published in 1891, one observer described a “luminous band extended east and west,” like a straight tail of a large comet. In 1933, aurora pioneer Carl Stormer snapped an early image of the occurrence in a black-and-white photo.

A few weeks after STEVE was formally identified in 2018, citizen-scientist Michael Hunnekuhl began searching through past studies and compiled a list of these historical and recent observations. He created a database of more than 1,000 observations, which he said is probably the largest STEVE database in the world.

STEVE has been observed in both hemispheres, including Antarctica. Areas with the highest number of days with reports are Canada, the United States, New Zealand and Australia, though that could partly be a result of high awareness of the phenomenon in those locations.

“The occurrence rate of Steve and observation rate can be different, especially if you go back a few years, because people were not aware of Steve,” said Hunnekuhl. The weather might influence reporting rates as well, he said, because people may be less likely to go outside in certain conditions.

Nonetheless, several trends have appeared in the data: STEVE appears most often in March and September near the equinoxes (which is also when aurora activity usually increases). The visible arc lasts about 30 minutes and very rarely after midnight.

Lach said she typically looks for STEVE when aurora activity is high from a solar storm. STEVE is often seen in conjunction with an aurora, although it is usually physically separated. It appears about 30 minutes after the aurora begins to brighten. Lach said she finds STEVE by looking off the western edge of the aurora. STEVE can be blocked out by a bright aurora, though, especially if it’s close to it.

STEVE can also take on slightly different forms. It can appear as just a short arc to the west of the aurora or extend across the full sky from east to west. Large manifestations also show more depth in color — a deep red on top, mauve in the middle and white on the bottom.

As the STEVE arc persists, a green picket fence may appear. Sometimes the arc disappears and the green picket fence remains.

How is STEVE different from the aurora?

STEVE and the aurora are both a ghostly light phenomenon in the atmosphere, but that’s about where the similarity ends.

NASA researcher Bea Gallardo-Lacourt, who has been part of STEVE research since the inaugural paper in 2018, sums up a few differences in their creation: Auroras involve a physical mechanism, with electrons and ions raining into our upper atmosphere and exciting atoms. The STEVE arc is a hot band of gas that involves a chemical reaction, which produces a glow.

Gallardo-Lacourt, who also hasn’t seen STEVE, explains that the arc is associated with a very fast stream of particles — about five times faster than seen in auroras. One proposed mechanism is that this strong plasma flow excites nitrogen molecules in the atmosphere that then interact with oxygen molecules, creating nitric oxide. This nitric oxide, which is otherwise rare in our atmosphere, is energized and glows, emitting light in the purple range of the visible light spectrum.

This NASA animation shows that STEVE (in purple) appears at lower latitudes than auroras (in green) over North America. (Video: NASA GSFC/CIL/Krystofer Kim)

Their sources also vary. The creation of an aurora begins when the sun sends a surge of energy to Earth, such as through an explosion on the sun called a coronal mass ejection, and spurs a large-scale geomagnetic storm. However, STEVE has occurred with and without geomagnetic storms.

Nishimura, a researcher at Boston University, said STEVE seems to always occur during local and brief disturbances in Earth’s magnetic field, called substorms. Substorms can occur without a large geomagnetic storm and happen daily. Yet despite the ubiquity of substorms, researchers are perplexed about why STEVE sightings are still relatively rare compared with auroras.

With every photo and report, people are understanding more about this relatively unexplored part of our atmosphere, and its connection with the sun. If you have seen STEVE, reach out to any of the researchers — or share your sightings with us on Twitter.