Geomagnetic interpretation of the solar dynamo

Analysis is made of the geomagnetic-activity aa  index covering solar cycle 11 to the beginning of 24, 1868– 2011. Autocorrelation shows 27.0-d recurrent geomagnetic activity that is well-known to be prominent during solar-cycle minima; some minima also exhibit a smaller amount of 13.5-d recurrence. Previous work has shown that the recent solar minimum 23– 24 exhibited 9.0 and 6.7-d recurrence in geomagnetic and heliospheric data, but those recurrence intervals were not prominently present during the preceding minima 21– 22 and 22– 23. Using annual-averages and solar-cycle averages of autocorrelations of the historical aa  data, we put these observations into a long-term perspective: none of the 12 minima preceding 23– 24 exhibited prominent 9.0 and 6.7-d geomagnetic activity recurrence. We show that the detection of these recurrence intervals can be traced to an unusual combination of sectorial spherical-harmonic structure in the solar magnetic field and anomalously low sunspot number. We speculate that 9.0 and 6.7-d recurrence is related to transient large-scale, low-latitude organization of the solar dynamo, such as seen in some numerical simulations. As time permits, other results related to the geomagnetic spectrum will be discussed.