Figure 1

Time evolution of sunspot number, area latitudes and extreme and recurrent geomagnetic activity. Grey shading indicates the quiet interval between switch-off and switch-on on all panels. Panel (a): 13 month smoothed SSN (red) and 40 year smoothed SSN (blue). Black crosses are at times 12 months following the smoothed SSN downcrossing of the 40 year smoothed SSN, and at the smoothed SSN upcrossing of the slow trend, providing estimates of the switch-off and switch-on times respectively. Panel (b): active region areas in single Carrington rotation-latitude bins (black) overplotted with the area centroid for each Carrington rotation (yellow). Panel (c) overplots on all sunspot area latitude bins with area > 50 μ Hem the Carrington rotation during which the aa index exceeds a threshold of 100 (blue) 200 (green) 300 (orange) 400 (red) 500 (purple) 600 (black) nT. Horizontal blue lines on panels (b,c) indicate latitudes \(\pm 15^{\circ }\). Panel (d): the aa geomagnetic index, blue line indicates aa = 300 nT. Panel (e) aa index autocovariance at a lag of 27 days, approximately the solar rotation (black) and at 10 days (red), to provide an indication of the level of ‘by chance’ autocovariance, this is exceeded by the horizontal blue line at 0.25.