Chromospheric extension of the MURaM code

Abstract

Context. Detailed numerical models of the chromosphere and corona are required to understand the heating of the solar atmosphere. An accurate treatment of the solar chromosphere is complicated by the effects arising from non-local thermodynamic equilibrium (NLTE) radiative transfer. A small number of strong, highly scattering lines dominate the cooling and heating in the chromosphere. Additionally, the recombination times of ionised hydrogen are longer than the dynamical timescales, requiring a non-equilibrium (NE) treatment of hydrogen ionisation. Aims. We describe a set of necessary additions to the MURaM code that allow it to handle some of the important NLTE effects. We investigate the impact on solar chromosphere models caused by NLTE and NE effects in radiation magnetohydrodynamic simulations of the solar atmosphere. Methods. The MURaM code was extended to include the physical process required for an accurate simulation of the solar chromosphere, as implemented in the Bifrost code. This includes a time-dependent treatment of hydrogen ionisation, a scattering multi-group radiation transfer scheme, and approximations for NLTE radiative cooling. Results. The inclusion of NE and NLTE physics has a large impact on the structure of the chromosphere; the NE treatment of hydrogen ionisation leads to a higher ionisation fraction and enhanced populations in the first excited state throughout cold inter-shock regions of the chromosphere. Additionally, this prevents hydrogen ionisation from buffering energy fluctuations, leading to hotter shocks and cooler inter-shock regions. The hydrogen populations in the ground and first excited state are enhanced by 102–103 in the upper chromosphere and by up to 109 near the transition region. Conclusions. Including the necessary NLTE physics leads to significant differences in chromospheric structure and dynamics. The thermodynamics and hydrogen populations calculated using the extended version of the MURaM code are consistent with previous non-equilibrium simulations. The electron number and temperature calculated using the non-equilibrium treatment of the chromosphere are required to accurately synthesise chromospheric spectral lines.

A Comprehensive Radiative Magnetohydrodynamics Simulation of Active Region Scale Flux Emergence from the Convection Zone to the Corona

“…Radiation of plasma hotter than 2 × 10 4 K is modeled by the optically thin radiative loss. A more sophisticated treatment that considers non-LTE effects in the chromosphere, such as in the BIFROST code and a new version of the MURaM code (Przybylski et al 2022), would be computationally too demanding for simulations on the spatial and temporal scales presented here. The effects of partial/nonequilibrium ionization and ion-neutral interactions (Cheung & Cameron 2012;Danilovic 2017;Khomenko et al 2018;Martínez-Sykora et al 2012, 2020 are not accounted for in our simulation.…”

MPI-AMRVAC 3.0: Updates to an open-source simulation framework

“…they are especially concerned with more advanced radiative transfer aspects, going beyond local thermodynamic equilibrium and approximating the complex chromospheric radiative aspects, as realized recently within MURaM, for example by Przybylski et al (2022). Future efforts toward such truly realistic radiative-MHD or radiative-multi-fluid models on evolving, block-AMR settings are highly desirable.…”

Numerical Simulations and Observations of Mg ii in the Solar Chromosphere

“…Effects that go beyond the standard MHD description of the chromospheric plasma, e.g., generalized Ohm's law (GOL; e.g., Khomenko et al 2018), including ambipolar diffusion and nonequilibrium (NEQ) ionization of H and He will also alter the opacity of the Mg II lines, by altering the heating and electron density profiles of the chromosphere and thus potentially the intensity and width of the emergent lines Bose et al 2021;Przybylski et al 2022). There are other smaller-scale candidate physical processes for heating the chromosphere as well: the thermal Farley-Buneman instability (TFBI; Oppenheim et al 2020).…”