A New Approach in Understanding Growth and Decay of the Sunspots

TL;DR

This paper presents a new theoretical model explaining sunspot growth and decay based on their formation depths, rotational gradients, and magnetic forces, aligning well with observed sunspot area evolution.

Contribution

It introduces a novel analytical approach linking sunspot area changes to rotational gradients and magnetic forces at different depths, improving understanding of sunspot dynamics.

Findings

Growth rate depends on Lorentzian force fluctuations and meridional flow velocity.

Decay rate is mainly influenced by magnetic diffusivity and rotational gradient.

Theoretical curves match observed sunspot area evolution reasonably well.

Abstract

From the previous study (Hiremath 2009b; Hiremath 2010), on the genesis of solar cycle and activity phenomena, it is understood that sunspots are formed at different depths by superposition of Alfven wave perturbations of a strong toroidal field structure in the convective envelope and after attaining a critical strength, due to buoyancy, raise toward the surface along the rotational isocontours that have positive (0.7-0.935 $R_{\odot}$) and negative (0.935-1.0 $R_{\odot}$) rotational gradients. Owing to physical conditions in these two rotational gradients, from the equation of magnetic induction, sunspot's area growth and decay problem is solved separately. It is found that rate of growth of sunspot's area during its evolution at different depths is function of steady and fluctuating parts of Lorentzian force of the ambient medium, fluctuations in meridional flow velocity, radial variation of rotational gradient and $cot(\vartheta)$ (where $\vartheta$ is co-latitude). While rate of decay of sunspot's area at different depths during its evolution mainly depends upon magnetic diffusivity, rotational gradient and $sin^{2}(\vartheta)$. Gist of this study is that growth and decay of area of the sunspot mainly depends upon whether sunspot is originated in the region of either positive or negative rotational gradient. For different latitudes and life spans of the sunspots on the surface during their evolutionary history, both the analytically derived theoretical area growth and decay curves match reasonably well with the observed area growth and decay curves.