  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.
