Wings of the butterfly: Sunspot groups for 1826–2015^⋆

¹ Space Climate Research Unit, University of Oulu, 90014 Oulu, Finland
e-mail: Ilya.Usoskin@oulu.fi
² Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland
³ Leibniz Institute for Astrophysics Potsdam (AIP), 14482 Potsdam, Germany
⁴ Institut für Physik und Astronomie, Universität Potsdam, 14476 Potsdam, Germany

Received: 16 August 2016
Accepted: 9 December 2016

Abstract

The spatio-temporal evolution of sunspot activity, the so-called Maunder butterfly diagram, has been continously available since 1874 using data from the Royal Greenwich Observatory, extended by SOON network data after 1976. Here we present a new extended butterfly diagram of sunspot group occurrence since 1826, using the recently digitized data from Schwabe (1826–1867) and Spörer (1866–1880). The wings of the diagram are separated using a recently developed method based on an analysis of long gaps in sunspot group occurrence in different latitude bands. We define characteristic latitudes, corresponding to the start, end, and the largest extent of the wings (the F, L, and H latitudes). The H latitudes (30°–45°) are highly significantly correlated with the strength of the wings (quantified by the total sum of the monthly numbers of sunspot groups). The F latitudes (20°–30°) depict a weak tendency, especially in the southern hemisphere, to follow the wing strength. The L latitudes (2°–10°) show no clear relation to the wing strength. Overall, stronger cycle wings tend to start at higher latitudes and have a greater wing extent. A strong (5–6)-cycle periodic oscillation is found in the start and end times of the wings and in the overlap and gaps between successive wings of one hemisphere. While the average wing overlap is zero in the southern hemisphere, it is two to three months in the north. A marginally significant oscillation of about ten solar cycles is found in the asymmetry of the L latitudes. The new long database of butterfly wings provides new observational constraints to solar dynamo models that discuss the spatio-temporal distribution of sunspot occurrence over the solar cycle and longer.