In this work, we present the development of the Tonalli code: a simplified multi-species (HI, HII, H-, and e) magnetohydrodynamics (MHD) model in Non-local Thermodynamic Equilibrium (NLTE) focused on solar chromospheric conditions. This new model integrates two well established models, Newtonian CAFE (MHD) and PakalMPI (NLTE), through a self-convergence system that links the state equations used by both codes to calculate density, pressure and temperature, with the mean molecular weight (μ) serving as a proxy. Newtonian CAFE computes the plasma variables using the ideal MHD while PakalMPI calculates the species densities of neutral Hydrogen (HI), protons (HII), negative Hydrogen (H-), and electrons (ne) under the NLTE approximation. We used Tonalli to test the stability of the hydrostatic C7 model, covering 3000 km of the solar chromosphere with a vertical constant field of 30 Gauss and a vertical constant gravity field of 274. 0 ms-2. As a result, Tonalli generates 3D cubes of densities, temperature, pressure, mean molecular weight, and the departure coefficient of Hydrogen in its first energy level b1, providing a detailed representation of the ionization states of the plasma at chromosphere altitudes. Despite the fact that MHD conditions can lead to numerical diffusion of the plasma, we demonstrate robustness and consistency, and self-convergence of the model with the relative error of electron density reaching values of 3. 7x10^-7.
Huipe-Domratcheva et al. (2025) studied this question.