The 22‐year cycle of geomagnetic and solar wind activity

Abstract

The 22‐year cycle in geomagnetic activity is characterized by high activity during the second half of even‐numbered solar cycles and the first half of odd‐numbered cycles. We present new evidence for this 22‐year cycle using the aa magnetic index for the years 1844–1994. Over this 150‐year interval, the 22‐year cycle can be observed through differences between the decay phases of even‐ and odd‐numbered cycles in (1) average values of a 27‐day recurrence index; (2) the results of a χ2 “event” analysis of 27‐day recurrences of both disturbed and quiet days; and (3) an apparent annual modulation of the 27‐day peak in the power spectrum of the aa index. Currently, the 22‐year variation is attributed to the Russell‐McPherron solar wind ‐ magnetosphere coupling mechanism working in conjunction with the Rosenberg‐Coleman polarity effect. Contrary to this viewpoint, we argue that an intrinsic 22‐year solar variation (other than polarity reversal), revealed in the systematic low‐high alternation of even‐odd sunspot maxima within the last six complete Hale cycles, is the dominant cause of the 22‐year cycle in geomagnetic activity. This sunspot and related coronal mass ejection variation should lead directly to higher geomagnetic activity during the first‐half of odd‐numbered solar cycles. Various lines of evidence (including 1–3 above) indicate that 27‐day recurrent wind streams are more prominent during the decline of even‐numbered solar cycles, contributing to the higher geomagnetic activity observed at those times. These stronger recurrence patterns may be related to the more rapid expansion of polar coronal holes (faster movement of the coronal streamer belt to low latitudes) observed following the maxima of recent even‐numbered cycles. The amplitudes of the 22‐year sunspot and geomagnetic activity cycles over the last 150 years are shown to be highly correlated. The 22‐year pattern of geomagnetic activity appears to be a reflection of the solar dynamo coupling of poloidal magnetic fields on the decline of one solar cycle to the toroidal fields at the maximum of the following cycle. It seems likely that the 22‐year variation in sunspot/solar wind activity plays a role in the observed 22‐year modulation of galactic cosmic ray intensity.