Three-dimensional modeling of chromospheric spectral lines in a simulated active region

Friday, May 3, 2019

Recently, the radiative magnetohydrodynamic (R-MHD) code MURaM was extended to include the corona. The code was used to simulate a bipolar active region with additional parasitic flux emergence near one of the sunspots that produced a flare.

Image of Simulated images of Ca II 8542 Å, Ca II K, Mg II k, and H
Simulated images of Ca II 8542 Å, Ca II K, Mg II k, and H from the model atmosphere at the disk center (=1.0). Upper row: are taken at the nominal line center. Bottom row: is the extracted intensity at the maximum formation height. The images given in brightness temperature Tb and are clipped at 3.6-7 kK for 8542 Å, 3.6-7 kK for Ca II K, 3.6-8 kK for Mg II k, and 3.6-10 kK for H-alpha. At the top left of each panel we show the minimum, maximum, and average brightness temperature Tb for the FOV.

While the simulation focused on coronal behavior, it did have a simplified chromosphere. We aim to model the H-alpha, Mg II HK, Ca II HK, and Ca II 8542 A lines in a 3D model atmosphere containing an active region, and we investigate how it appears in synthetic images and spectra. We have modeled the spectral lines using the 3D non-LTE radiative transfer code Multi3D. To obtain non-LTE electron densities, we solve the statistical equilibrium equations for hydrogen simultaneously with the charge conservation equation. We treat the Ca II K and Mg II k with partially coherent scattering. This simulation reproduces long fibrils that span between the opposite-polarity sunspots. Magnetic field lines are aligned with the H-alpha fibrils. The latter holds to a lesser extent for the other lines. The simulation shows structures in the H-alpha line core that look like flare ribbons, they are less visible in the other line cores. The emission in the ribbons is caused by a dense chromosphere and a transition region at high column mass. The Mg II k and Ca II K lines show broad single emission peak caused by a deep onset of the chromospheric temperature rise. The simulation produces long fibrils similar to what is seen in observations. It also produces structures similar to flare ribbons despite the lack of non-thermal electrons in the simulation. The latter suggests that thermal conduction might be a significant agent in transporting flare energy to the chromosphere in addition to non-thermal electrons.

First HAO Author's Name: Matthias Rempel

Publication Name: Astronomy and Astrophysics