Most of us have heard of the Aurora Borealis, otherwise known as the Northern Lights. Every year people travel to remote northern locations in the Yukon, Alaska, Greenland and Iceland to see this magnificent display of lights.
There is, however, another atmospheric light show that you may be less familiar with: STEVE. Short for Strong Thermal Emission Velocity Enhancement, this strange aurora has puzzled scientists for years. Finally, they are answering some of their questions, and it turns out STEVE is even more atypical than they thought.
What is an Aurora?
An aurora is a result of gaseous particles in the earth’s atmosphere colliding with charged particles that the sun releases from its own atmosphere. In the northern hemisphere, this is called an aurora borealis, and in the southern hemisphere, it’s called an aurora australis [1,2].
The magnetic field around the earth shields us from most of the sun’s energy and particles. The amount of energy the sun sends out, however, is not consistent. While there is a constant stream of solar energy, occasionally there are solar storms. Scientists call one type of these storms a coronal mass ejection.
During a coronal mass ejection, the sun sends out a large bubble of electrified gas that travels through space and extremely high speeds. As it speeds toward the earth, some of its energy travels down the lines of our magnetic field at the north and south poles. It then enters the earth’s atmosphere .
Once it is in our atmosphere, it interacts with the gases contained within it and causes them to glow. Different gases create different colors. Oxygen results in a pale yellowish-green color, which is the most common. Oxygen at very high altitudes (up to two hundred miles above the earth’s surface) will create an extremely rare all-red aurora. Nitrogen produces a blue or purple-ish red aurora .
What is STEVE?
STEVE looks and behaves a lot like a typical aurora, however, there are some key differences. For this reason, this strange celestial phenomenon has intrigued scientists for years.
In the northern hemisphere, you can see STEVE in locations further south than the Northern Lights. Instead of the typical green color, it appears as a ribbon of pink or mauve, often with green columns of light passing through the ribbon. Scientists often refer to this as STEVE’s “picket fence”.
Initially, scientists weren’t sure if STEVE was some kind of aurora. In 2018, however, one study revealed that it is not due to charged particles raining down into our upper atmosphere, as is the case with typical auroras.
To make it even more complicated, the study also determined that STEVE can appear at the same time as the most powerful aural lights .
A later 2019 study determined that the STEVE’s mauve streak and green picket fence are actually a result of two distinct phenomena from two separate processes. They found that the mauve arch occurs when charged particles are heated high up in the earth’s atmosphere. The green picket fence, on the other hand, is more similar to an aurora .
“Aurora is defined by particle precipitation, electrons and protons actually falling into our atmosphere, whereas the STEVE atmospheric glow comes from heating without particle precipitation,” said study co-author and space physicist Bea Gallardo-Lacourt. “The precipitating electrons that cause the green picket fence are thus aurora, though this occurs outside the auroral zone, so it’s indeed unique.” 
STEVE is More Complicated Than We Thought
Fast-forward to 2020, and scientists are now questioning the results of this research.
Specifically, new research has shown that the green picket fence is not an aurora after all. While the green streaks do contain glowing oxygen, a lack of nitrogen emissions suggest that it is not the same as an aurora.
What’s more, researchers along with citizen scientists have identified another strange aspect of the picket fence. Using satellite along with high-resolution ground images, they have noticed small green streaks. They stick out like feet from the bottom of some of the vertical stripes.
The researchers say that the electron showers that cause auroras cannot be responsible for these streaks. Joshua Semeter, an engineer at Boston University, says that they don’t really know what’s going on just yet. They believe, however, that these horizontal streaks must come from some process that is specific to STEVE .
What is Causing the Streaks?
Semeter and his fellow researchers suggest in their study that these streaks only appeared that way because of motion blur and radiative lifetime effects. This occurs when spherical blobs of glowing gas move through the atmosphere.
Semeter says that one reason these blobs might occur is because of turbulence in the plasma. This turbulence is what creates STEVE’s purple band. Positively charged atoms in the plasma can travel through the atmosphere easily and form a smooth purple arch. The electrons in the plasma, however, are far lighter. For this reason, the turbulence is more likely to affect them.
These high-energy electrons can get tangled up in the whirling air on the edge of the plasma stream beneath the purple streak. These particles can then excite the oxygen molecules and cause them to glow green.
Further Research Required
This, however, is just a theory.
“We found events where these little feet appear before or at the same time as the green column above it,” says Semeter. “It appears that the green emission is actually expanding upward along the magnetic field line.” 
Right now, the researchers need to confirm whether electrons from the magnetosphere are pouring into the atmosphere in the area where there is a STEVE picket fence. If future satellite observations don’t see these electron showers, that will provide further evidence to support Semeter’s theory.
For now, however, we can just enjoy that mysterious beauty of this celestial phenomenon.
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