What are these green pickets? A team of researchers led by Toshi Nishimura of Boston University may have found the answer. Using data from NASA's THEMIS spacecraft, they pinpointed STEVE's power source. More than 22,000 km above Earth's surface, magnetic explosions called "substorms" hurl streams of hot plasma toward Earth. When the streams reach an altitude ~250 km above Earth's surface, they begin to emit a soft-purple light. This is STEVE.
The same explosions spray energetic electrons toward Earth. The electrons move even deeper into the atmosphere, all the way down to 100 km, where they ignite fingers of green auroras. This is the picket fence.
So when you see STEVE, look for his sidekick as well. Sometimes the picket fence appears first, so one can herald the other. Either way, it's a beautiful show.