We use wavelets and Lomb–Scargle spectral analysis techniques to investigate the changing pattern of the different harmonics of the 27-day solar rotation period of the AE index during various phases of different solar cycles between 1960 and 2014. Results show that, particularly during the solar minimum of 23–24 in 2008, the 27-day period is no longer detectable above the 95 % CI, while auroral geomagnetic activity is now dominated by the second (13.5-day) and third (9.0-day) harmonics.

In this study we performed a wavelet and Lomb-Scargle analysis of the geomagnetic activity K-index at Lerwick (LER), Hermanus (HER) and Canberra (CNB) for the period between 1960 and 2010, overlapping with solar cycles 20 to 23. Daily mean K indices are used to identify how several harmonics of the 27.0-day recurrent period change during each solar cycle when comparing high- and mid-latitude geomagnetic activity, applying a 95% confidence level.