1. Introduction
1.1. General information
1.1.1. What is PORTA
PORTA is a modular computer program for solving three-dimensional (3D), non-equilibrium radiative transfer problems with massively parallel computers (see Štěpán, J., and Trujillo Bueno, J. 2013 [1]). The present version can be applied for modeling the spectral line polarization produced by the scattering of anisotropic radiation and the Hanle and Zeeman effects assuming complete frequency redistribution, either using two-level or multilevel atomic models. The numerical method of solution used to find the self-consistent values of the atomic density matrix at each point of the model’s Cartesian grid is based on Jacobi iterative scheme and on a short-characteristics formal solver of the Stokes-vector transfer equation that uses monotonic Bézier interpolation. The code can also be applied for computing the linear polarization of the continuum radiation caused by Rayleigh and Thomson scattering in 3D models of stellar atmospheres. It can also be applied to solve the simpler 3D radiative transfer problem of unpolarized radiation in multilevel systems. The present public version of PORTA comes with the HDF5 input/output, an advanced graphical user interface, and is released within the framework of the POLMAG project funded by the European Research Council (see the POLMAG webpage).
1.1.2. Purpose of the code
In addition to a 3D radiative transfer code itself suitable for doing applications, PORTA provides a flexible framework for further developments. This is because PORTA has been originally developed in a modular way, with the aim of facilitating the preparation of new modules for solving a broader set of radiative transfer problems in the future (e.g., the PRD problem in 3D models). The developer manual will be included in one of the future releases of the code, but the users are already encouraged to share their own modules with the community via our repository.
PORTA is provided with several tools written in C and Python for creating the models, visualization of the results, and producing publication-level quality figures.
1.1.3. Structure of the code
The code consists of three main parts:
The PORTA wrapper: It runs the main loop of the code and executes the user scripts.
PORTAL: The library which contains all the functionalities needed for solving the radiative transfer problem, most of the I/O operations, parallelization, and the Application Programming Interface (API) necessary for most of the polarized radiative transfer applications (including the common mathematical and physical functions).
Modules: The problem-specific libraries implementing the physics of the matter-radiation interaction in different physical regimes. Learn more about the modules in section Modules.
1.1.4. Note on implementation and parallelism
The code has been written in the C programming language and it is parallelized via MPI libraries. In the current version, the parallelization is done using domain decomposition in the Z coordinate and in the radiation frequency domain. This design leads to two main advanatages: firstly, in typical problems the computing time scales almost linearly with the number of CPU cores, i.e., twice the number of CPU cores means 50% less wall-clock time. The parallelization strategy does not affect the convergence rate. Secondly, the accuracy of the solution is identical for all possible parallelizations. In the future, we plan to extend the parallelization in the spatial domain in order to avoid possible memory issues when using extremely large 3D models.
1.2. Terms of use
You can use PORTA freely in your scientific research.
In case you publicly present your results obtained with the help of the code, we ask you to quote the paper Štěpán, J., and Trujillo Bueno, J. 2013, A&A, 557, A143, and to include a link to the code repository https://gitlab.com/polmag/PORTA.