Co-rotating Interaction Region (CIR)

A turbulent, compressed zone that forms in interplanetary space where a fast stream of solar wind, flowing out from a coronal hole, catches up to and plows into slower solar wind ahead of it. Because both streams rotate around the Sun together with it, the resulting compression region also co-rotates, sweeping past Earth repeatedly with roughly the Sun's 27-day rotation period. CIRs typically produce milder, longer-lasting geomagnetic disturbances than a coronal mass ejection, but their recurring nature makes them a reliable, predictable source of minor-to-moderate space weather activity.