From the American Geophysical Union weekly highlights:
- Minimum 23-24 showed recurrence intervals of 9.0 and 6.7-d
- Historical geomagnetic activity data show that minimum 23-24 was unusual
- The heliosphere during minimum 23-24 had unusual sectorial structure
Since the mid-1800s, scientists have been systematically measuring changes in the Earth’s magnetic field and the occurrence of geomagnetic activity. Such long- term investigation has uncovered a number of cyclical changes, including a signal associated with 27-day solar rotation. This is most clearly seen during the declining phase and minimum of each 11-year solar cycle, when the Sun’s magnetic dipole is sometimes tilted with respect to the Sun’s rotational axis. With the Sun’s rotation and the emission of solar wind along field lines from either end of the solar magnetic dipole, an outward propagating spiral-like pattern is formed in the solar wind and the interplanetary magnetic field that can drive 27-day, and occasionally 13.5-day, recurrent geomagnetic activity. Recurrent geomagnetic activity can also be driven by isolated and semipersistent coronal holes, from which concentrated streams of solar wind can be emitted.
During the most recent solar minimum, which took place from 2006 to 2010, however, several researcher groups noticed 6.7-day and 9-day recurrent changes in geomagnetic activity, and similar patterns in the interplanetary magnetic field, and the solar wind.
Using modern data covering the previous two solar minima, these higher-frequency occurrences were judged to be unusual. Love et al. analyzed historical geomagnetic activity records from 1868 to 2011 and find that the 6.7-day and 9-day recurrent changes were actually unique in the past 140 years. They suggest that the higher-frequency changes in geomagnetic activity are due to an unusual transient asymmetry in the solar dynamo, the turbulent, rotating plasma deep within the sun which generates the magnetic field.
Source: Geophysical Research Letters, doi:10.1029/2011GL050702, 2012
Title: Geomagnetic detection of the sectorial solar magnetic field and the historical peculiarity of minimum 23-24
Authors: Jeffrey J. Love and E. Joshua Rigler: Geomagnetism Program, U.S. Geological Survey, Denver, Colorado, USA;
Sarah E. Gibson: High Altitude Observatory, NCAR, Boulder, Colorado, USA.
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.