Photographers had known about it for decades, but scientists didn’t get wind of it until 2016. It’s a streak, purple or white, across the night sky. It looks like it could be a type of aurora… but that’s not what it is.
The kind of aurora that we’re familiar with happen when charged particles (electrons and photons) from the sun hit neutral atoms in the Earth’s ionosphere. These collisions excite molecules in our atmosphere, bringing them to higher energy levels. When these charged particles drop down to lower energy states, they release light. This creates the bright streaks that we picture when we think of aurora. The specific colors depends on which gases are being excited. For example, oxygen gives off green and Nitrogen can give off blue or red. In addition, aurora can only be seen near the poles, and are visible every night (with the right viewing conditions).
The new things people were seeing, the purple streaks, do not behave like this. They show up near the equator, and only a few times a year. We also know that they aren’t excited protons, because they show up on equipment that wouldn’t otherwise be able to detect proton wavelengths.
The observers who brought it to researchers’ attention call it Steve, referring to the movie “Over the Hedge.” Scientists decided to turn it into an acronym. Now, the phenomenon is called STEVE, for Strong Thermal Emission Velocity Enhancement.
So we know it’s not aurora. But what is it?
Researchers from the University of Calgary wanted to see if it was a similar mechanism to aurora–if it was from particle interaction in the ionosphere. To figure it out, they analyzed pictures from a STEVE event in 2008. They also looked at data from a satellite called POES-17, which can measure charged particles raining down to the ionosphere. The satellite had no record of charged particles entering the ionosphere while the event was occurring, which means that STEVE happens for different reasons. What those specific reasons are have yet to be determined.
For now, that is the only science out about this event. As of now, the best language researchers have to describe this is “skyglow.” In future studies, they want to find out if the light is coming from the ionosphere, or higher up.