THE 22-YEAR CYCLE OF GEOMAGNETIC AND SOLAR-WIND ACTIVITY

The 22-year cycle in geomagnetic activity is characterized by high act ivity during the second half of even-numbered solar cycles and the fir st half of odd-numbered cycles. We present new evidence for this 22-ye ar cycle using the aa magnetic index for the years 1844-1994. Over thi s 150-year interval, the 22-year cycle can be observed through differe nces 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 chi( 2) ''event'' analysis of 27-day recurrences of both disturbed and quie t days; and (3) an apparent annual modulation of the 27-day peak in th e power spectrum of the aa index. Currently, the 22-year variation is attributed to the Russell-McPherron solar wind - magnetosphere couplin g mechanism working in conjunction with the Rosenberg-Coleman polarity effect. Contrary to this viewpoint, we argue that an intrinsic 22-yea r solar variation (other than polarity reversal), revealed in the syst ematic 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 ejecti on variation should lead directly to higher geomagnetic activity durin g the first-half of odd-numbered solar cycles. Various lines of eviden ce (including 1-3 above) indicate that 2?;day recurrent wind streams a re more prominent during the decline of even-numbered solar cycles, co ntributing to the higher geomagnetic activity observed at those times. These stronger recurrence patterns may be related to the more rapid e xpansion of polar coronal holes (faster movement of the coronal stream er belt to low latitudes) observed following the maxima of recent even -numbered cycles, The amplitudes of the 22-year sunspot and geomagneti c activity cycles over the last 150 years are shown to be highly corre lated. The 22-year pattern of geomagnetic activity appears to be a ref lection of the solar dynamo coupling of poloidal magnetic fields on th e 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 su nspot/solar wind activity plays a role in the observed 22-year modulat ion of galactic cosmic ray intensity.