Dear Colleagues,

The new Solar Cycle 25 has started, and already some scientifically interesting solar events have been observed in various wavelength regimes and through various instruments. We can expect increased activity in the field of solar observation in the coming years. This is mainly due to the fact that the last solar cycle, Solar Cycle 24, was weaker than the previous ones, and there has been some discussion that the weaker trend will continue, and that the current cycle is even weaker than the last one. What could be behind this? Is there a specific reason or is this just a statistical variation? Other fundamental questions also remain, involving solar cyclicity—for instance, the exact reason for the 11-year solar cycle, which is yet to be identified.

Solar cyclicity can be investigated via both observational and theoretical approaches. The sunspot number has been the primary indicator for solar activity. However, there are also several other indicators, whose potential usefulness, especially in the context of the descending solar cycle trend, is of interest. Modeling could also provide useful information for solar cyclicity studies with new, powerful simulation tools.

The main purpose of this Special Issue is to gather versatile investigations of solar cyclicity which are based on either observations or more theoretical approaches. We encourage authors to submit original research articles to this Special Issue of the journal that could shine light on new information to understand the solar cycle.

Dr. Juha Kallunki
Prof. Dr. Arnold Hanslmeie
Guest Editors

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• solar cycle
• solar activity
• solar cyclicity
• solar observations
• simulations
• modeling