Looking for granulation and periodicity imprints in the sunspot time series

TL;DR

This study introduces a novel wavelet-based spectral analysis of sunspot data, revealing a 22.67-day periodicity potentially linked to solar convection, alongside the known 22-year cycle and daily volatility.

Contribution

A new wavelet spectral method uncovers multiple periodicities in sunspot activity, including a previously unclear 22.67-day cycle, and analyzes hemispheric asymmetries.

Findings

Identified a 22.67-day periodicity in sunspot data.

Confirmed an 18.43-year solar magnetic cycle.

Detected hemispheric asymmetries in periodic sources.

Abstract

The sunspot activity is the end result of the cyclic destruction and regeneration of magnetic fields by the dynamo action. We propose a new method to analyze the daily sunspot areas data recorded since 1874. By computing the power spectral density of daily data series using the Mexican hat wavelet, we found a power spectrum with a well-defined shape, characterized by three features. The first term is the 22 yr solar magnetic cycle, estimated in our work to be of 18.43 yr. The second term is related to the daily volatility of sunspots. This term is most likely produced by the turbulent motions linked to the solar granulation. The last term corresponds to a periodic source associated with the solar magnetic activity, for which the maximum of power spectral density occurs at 22.67 days. This value is part of the 22-27 day periodicity region that shows an above-average intensity in the power spectra. The origin of this 22.67 day periodic process is not clearly identified, and there is a possibility that it can be produced by convective flows inside the star. The study clearly shows a north-south asymmetry. The 18.43 yr periodical source is correlated between the two hemispheres, but the 22.67 day one is not correlated. It is shown that towards the large timescales an excess occurs in the northern hemisphere, especially near the previous two periodic sources. To further investigate the 22.67 day periodicity we made a Lomb-Scargle spectral analysis. The study suggests that this periodicity is distinct from others found nearby.