Signature of extended solar cycles as detected from Ca ii K synoptic maps of Kodaikanal and Mount Wilson Observatory

Abstract

In recent years there has been a resurgence of the study of extended solar cycles (ESCs) through observational proxies mainly in extreme ultraviolet. But most of them are limited only to the space-based era covering only about two solar cycles. Long-term historical data sets are worth examining for the consistency of ESCs. The Kodaikanal Solar Observatory (KSO) and the Mount Wilson Observatory (MWO) are two major sources of long-term Ca ii K digitized spectroheliograms covering the temporal spans of 1907–2007 and 1915–1985 respectively. In this study, we detected supergranule boundaries, commonly known as networks, using the Carrington maps from both KSO and MWO data sets. Subsequently we excluded the plage areas to consider only the quiet Sun (QS) and detected small-scale bright features through intensity thresholding over the QS network. Latitudinal density of those features, which we named "Network Bright Elements," could clearly depict the existence of overlapping cycles with equatorward branches starting at latitude ≈55° and taking about 15 ± 1 yr to reach the equator. We performed a superposed epoch analysis to depict the similarity of those extended cycles. Knowledge of such equatorward band interaction, for several cycles, may provide critical constraints on solar dynamo models.