Ambipolar diffusion in the Bifrost code

Abstract

Ambipolar diffusion is a physical mechanism related to the drift between charged and neutral particles in a partially ionized plasma that is key in many different astrophysical systems. However, understanding its effects is challenging due to basic uncertainties concerning relevant microphysical aspects and the strong constraints it imposes on the numerical modeling. Our aim is to introduce a numerical tool that allows us to address complex problems involving ambipolar diffusion in which, additionally, departures from ionization equilibrium are important or high resolution is needed. The primary application of this tool is for solar atmosphere calculations, but the methods and results presented here may also have a potential impact on other astrophysical systems. We have developed a new module for the stellar atmosphere Bifrost code that improves its computational capabilities of the ambipolar diffusion term in the Generalized Ohm's Law. This module includes, among other things, collision terms adequate to processes in the coolest regions in the solar chromosphere. As a key feature of the module, we have implemented the Super Time-Stepping (STS) technique, that allows an important acceleration of the calculations. We have also introduced hyperdiffusion terms to guarantee the stability of the code. We show that to have an accurate value for the ambipolar diffusion coefficient in the solar atmosphere it is necessary to include as atomic elements in the equation of state not only hydrogen and helium but also the main electron donors like sodium, silicon and potassium. In addition, we establish a range of criteria to set up an automatic selection of the free parameters of the STS method that guarantees the best performance, optimizing the stability and speed for the ambipolar diffusion calculations. We validate the STS implementation by comparison with a self-similar analytical solution.

Accepted in A&A, 10 pages, 7 figures

References in corpus (15)

• Global simulations of protoplanetary disks with ohmic resistivity and ambipolar diffusion
• Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex
• On the generation of solar spicules and Alfvénic waves
• Radiation Magnetohydrodynamic Simulations of Protostellar Collapse: Non-Ideal Magnetohydrodynamic Effects and Early Formation of Circumstellar Disks
• Non-equilibrium hydrogen ionization in 2D simulations of the solar atmosphere
• Onset of Fast Reconnection in Hall Magnetohydrodynamics Mediated by the Plasmoid Instability
• Energy flux of Alfven waves in weakly ionized plasma
• Two-dimensional Radiative Magnetohydrodynamic Simulations of Partial Ionization in the Chromosphere. II. Dynamics and Energetics of the Low Solar Atmosphere
• Rayleigh-Taylor instability in prominences from numerical simulations including partial ionization effects
• A three-dimensional numerical method for modelling weakly ionized plasmas
• Ion-neutral interactions and non-equilibrium ionization in the solar chromosphere
• Three-dimensional Simulation of Magnetized Cloud Fragmentation Induced by Nonlinear Flows and Ambipolar Diffusion
• An Explicit Scheme for Incorporating Ambipolar Diffusion in a Magnetohydrodynamics Code
• Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes
• The challenges of modelling microphysics: ambipolar diffusion, chemistry, and cosmic rays in MHD shocks