The formation of CIRs at stream-stream interfaces and resultant geomagnetic activity

Corotating interaction regions (CIRs) are regions of compressed plasma formed at the leading edges of corotating high-speed solar wind streams originating in coronal holes as they interact with the preceding slow solar wind. Although particularly prominent features of the solar wind during the declining and minimum phases of the 11-year solar cycle, they may also be present at times of higher solar activity. We describe how CIRs are formed, and their geomagnetic effects, which principally result from brief, southward interplanetary magnetic field excursions associated with Alfven waves. Seasonal and long-term variations in these effects are briefly discussed.