Time-dependent properties of sunspot groups -- I. Lifetime and asymmetric evolution

TL;DR

This study analyzes over 130 years of sunspot group data using Bayesian models, revealing asymmetric growth and decay patterns with weak correlations to size and latitude, enhancing understanding of sunspot evolution.

Contribution

The paper introduces a Bayesian skew-normal modeling approach to characterize the asymmetric temporal evolution of sunspot groups over a long historical dataset.

Findings

Sunspot groups' evolution can be well modeled with skew-normal functions.

Significant asymmetry exists between growth and decay phases.

Weak correlation between skewness and sunspot size or latitude.

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 (MSH) 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.