PERIODICITY. “The CIRs seldom have fast shocks or continuous, strongly southward IMF Bz, and thus drive only moderate magnetospheric activity. CIRs, being associated with the coronal hole structure, also exhibit 27-day periodicity.” (CIR = corotating interaction regions); (IMF= interplanetary magnetic field).
https://link.springer.com/article/10.12942/lrsp-2007-1