Changes in Quasi-periodic Variations of Solar Photospheric Fields: Precursor to the Deep Solar Minimum in Cycle 23?

Possible precursor signatures in the quasi-periodic variations of solar photospheric fields were investigated in the build-up to one of the deepest solar minima experienced in the past 100 years. This unusual and deep solar minimum occurred between Solar Cycles 23 and 24. We used both wavelet and Fourier analysis to study the changes in the quasi-periodic variations of solar photospheric fields. Photospheric fields were derived using ground-based synoptic magnetograms spanning the period 1975.14 to 2009.86 and covering Solar Cycles 21, 22, and 23. A hemispheric asymmetry in the periodicities of the photospheric fields was seen only at latitudes above ± 45∘ when the data were divided into two parts based on a wavelet analysis: one prior to 1996 and the other after 1996. Furthermore, the hemispheric asymmetry was observed to be confined to the latitude range of 45∘ to 60∘. This can be attributed to the variations in polar surges that primarily depend on both the emergence of surface magnetic flux and varying solar-surface flows. The observed asymmetry along with the fact that both solar fields above ± 45∘ and micro-turbulence levels in the inner-heliosphere have been decreasing since the early- to mid-nineties (Janardhan et al. in Geophys. Res. Lett.382, 20108, 2011) suggest that around this time active changes occurred in the solar dynamo that governs the underlying basic processes in the Sun. These changes in turn probably initiated the build-up to the very deep solar minimum at the end of Cycle 23. The decline in fields above ± 45∘, for well over a solar cycle, would imply that weak polar fields have been generated in the past two successive solar cycles, viz. Cycles 22 and 23. A continuation of this declining trend beyond 22 years, if it occurs, will have serious implications for our current understanding of the solar dynamo.