STEVE--short for "Strong Thermal Emission Velocity Enhancement"--was long thought to be a type of aurora borealis. But it's not. Auroras appear when particles rain down from space. STEVE, on the other hand, does not require "rain." Instead, satellite measurements show that it is a ribbon of hot (3000°C) gas speeding through the upper reaches of Earth's magnetic field faster than 10,000 mph. The ribbon's purple hue is still a mystery; some research suggests the color comes from heated nitrogen, but the jury's still out..
Studies show that STEVE appears most often in weeks around equinoxes--that is, now. If you live at high latitudes, be alert for purple ribbons in the sky.