Narrowing the solar surface flux transport parameter space through nonlinear feedbacks
The surface flux transport (SFT) model describes the evolution of the Sun's large-scale photospheric magnetic field. While linear transport processes are relatively well constrained, nonlinear feedbacks such as tilt and latitude quenching remain less explored, despite their potential role in regulating the solar cycle. We aim to determine how nonlinear quenching mechanisms and flux decay influence the admissible parameter space of the SFT model, and to identify parameter combinations that reproduce the observed characteristics of the solar polar magnetic field. We extend the parameter-space optimisation by introducing analytic prescriptions for tilt quenching (TQ) and latitude quenching (LQ) in the source term, together with a tunable flux-decay term. The model is solved numerically over a grid of meridional flow speeds ($u_{0}$), surface diffusivities ($η$), and decay timescales ($τ$). Admissible solutions are defined by agreement with {observationally motivated constraints on the {polar-field minimum-to-extremum amplitude ratio}, reversal timing, and the latitude of the polar cap boundary. Both TQ and LQ reduce the admissible parameter domain, with LQ exerting the stronger influence. Their combined action produces a pronounced saturation ("ceiling") that limits axial dipole amplification. The inclusion of a finite flux-decay timescale ($τ\simeq 8$--$10$~yr) further narrows the admissible domains within the present parameterised source formulation, whereas the non-decaying case may lead to unrealistically persistent dipole fields in this modelling framework. Correlations between $u_{0}$, $η$, and $τ$ reveal a coupled balance between advection, diffusion, and phenomenological flux loss.