Properties of sunspot cycles and hemispheric wings since the 19th century

¹ ReSoLVE Centre of Excellence, Space Climate Research Unit, University of Oulu, 90017 Oulu, Finland
e-mail: raisa.leussu@oulu.fi
² Sodankylä Geophysical Observatory (Oulu unit), University of Oulu, 90017 Oulu, Finland
³ Leibniz Institute for Astrophysics Potsdam, 14482 Potsdam, Germany

Received: 18 February 2016
Accepted: 17 June 2016

Abstract

Aims. The latitudinal evolution of sunspot emergence over the course of the solar cycle, the so-called butterfly diagram, is a fundamental property of the solar dynamo. Here we present a study of the butterfly diagram of sunspot group occurrence for cycles 7–10 and 11–23 using data from a recently digitized sunspot drawings by Samuel Heinrich Schwabe in 1825–1867, and from RGO/USAF/NOAA(SOON) compilation of sunspot groups in 1874–2015.

Methods. We developed a new, robust method of hemispheric wing separation based on an analysis of long gaps in sunspot group occurrence in different latitude bands. The method makes it possible to ascribe each sunspot group to a certain wing (solar cycle and hemisphere), and separate the old and new cycle during their overlap. This allows for an improved study of solar cycles compared to the common way of separating the cycles.

Results. We separated each hemispheric wing of the butterfly diagram and analysed them with respect to the number of groups appearing in each wing, their lengths, hemispheric differences, and overlaps.

Conclusions. The overlaps of successive wings were found to be systematically longer in the northern hemisphere for cycles 7–10, but in the southern hemisphere for cycles 16–22. The occurrence of sunspot groups depicts a systematic long-term variation between the two hemispheres. During Schwabe time, the hemispheric asymmetry was north-dominated during cycle 9 and south-dominated during cycle 10.