Time-dependent properties of sunspot groups

I. Lifetime and asymmetric evolution

¹ Department of Astronomy, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
e-mail: e.forgacs-dajka@astro.elte.hu
² Department of Physics & Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
e-mail: dobos@jhu.edu
³ Department of Physics of Complex Systems, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁴ Plasma Dynamics Group, University of Sheffield, Hounsfield Road, Hicks Building, Sheffield S3 7RH, UK
e-mail: i.ballai@sheffield.ac.uk

Received: 5 March 2021
Accepted: 28 May 2021

Abstract

Aims. In this paper, we aim to study the time dependence of sunspot group areas in a large sample composed of various databases spanning over 130 years, used state-of-the-art statistical methods.

Methods. For a carefully selected but unbiased sample, we use Bayesian modelling to fit the temporal evolution of the combined umbral and penumbral area of spot groups with a skew-normal function to determine the existence of any asymmetry in spot growth or decay. Our primary selection criteria guaranteed that only spot groups with a well-defined maximum area were taken into account. We also analysed the covariance of the resulting model parameters and their correlations with the physical parameters of the sunspots and the ongoing solar cycle.

Results. Our results show that the temporal evolution of well-observed sunspot groups that reach at least 50 millionths of a solar hemisphere at their maximum can be fitted surprisingly well with our model. Furthermore, we show significant asymmetry – described by a skew parameter of fitted curves – between the growing and decaying phases of analysed sunspot groups. In addition, we found a weak correlation between the values of skew parameters and the maximum area of sunspot groups and their hemispherical latitude.