Feb. 3, 2021: What made the auroras of Feb. 2nd so good? It was a co-rotating interaction region (CIR). CIRs are transition zones between slow- and fast-moving streams of solar wind. Solar wind plasma piles up in these regions, creating density gradients and shock waves that can rock Earth’s magnetic field much like a coronal mass ejection (CME).

A CIR hit Earth on Feb. 2nd and “the lights were incredible–just fantastic,” reports aurora tour guide Marianne Bergli who witnessed the display from a beach near Tromsø, Norway:

“Every color in the heavens cycled through the sky,” she says. “It. Was. Amazing.”

CIRs are a way for the sun to spark auroras without explosive solar activity. All that’s required is a fast solar wind stream brushing up against a slower one. In the transition zone, thick rivulets of plasma press magnetic fields together, creating strong shock-like structures that mimic CMEs. Indeed, some forecasters refer to co-rotating interaction regions as “mini-CMEs.”

Says Bergli, “I am looking forward to the next one!”