Ca ii 854.2 nm in an enhanced network region simulated with MURaM-ChE

TL;DR

This study tests how well the chromospheric extension of MURaM (MURaM-ChE) can reproduce the spatially averaged Ca II $854.2$ nm line profile by separating the influences of isotopic splitting and atmospheric dynamics. Three forward-modeling RT runs are used: im with all calcium isotopes in NLTE, cm as a composite isotope blend, and sim with only the most abundant isotope, evaluated at disk center using RH1.5D in a 1.5D framework; the simulated enhanced network atmosphere tends to reproduce the observed line width and depth when dynamics are strong. The key finding is that isotopic splitting is required to reproduce the inverse-C-shaped bisector and red asymmetry, while the dynamic chromosphere largely accounts for the line width; the composite model provides a good approximation but has height-dependent limitations due to varying isotope population ratios. Overall, the results demonstrate that forward modeling which includes isotopic splitting and realistic chromospheric dynamics is necessary to match Ca II $854.2$ nm observations and to avoid biases in inversions that neglect isotope effects.

Abstract

The Ca ii 854.2 nm line is widely used to study the chromosphere of the Sun. In the quiet Sun, the spatially averaged line profile shows a red asymmetry and a redshift of the line center. It is known that the effect of isotopic splitting must be taken into account in the forward modeling to reproduce the observed asymmetry. So far, no numerical model could match an average observed line profile in terms of the line width and asymmetry. Our goal is to investigate how well a simulation computed with the chromospheric extension of the MURaM code (MURaM-ChE) reproduces the spatially averaged Ca ii 854.2 nm line profile. We aim to determine the contributions from the isotopic splitting versus the dynamics in the atmosphere to the resulting line width and asymmetry. We solve the radiative transfer problem three times, once considering only the most abundant isotope of calcium in the atmosphere, once taking six calcium isotopes into account, and finally using a single composite atom model. We find the forward modeled spatially and temporally averaged spectra to be in good agreement with an average observation of the quiet Sun. In order to match the observed line width, the simulated atmosphere must be sufficiently dynamic. The typical red asymmetry can only be reproduced by taking the isotopic splitting effect into account, as suggested in the literature.

Figures (8)

• Figure 1: Overview of intensity, and atmospheric properties at the formation height of the intensity profile minimum. Panel (a) shows the continuum intensity at $\lambda=500\, \mathrm{nm}$ as brightness temperature, panel (b) shows the intensity at the profile minimum $\lambda_{\mathrm{min}}$ of the Caii $\lambda854.2$ nm spectral line as brightness temperature, panel (c) shows the temperature at the formation height of the line profile minimum, panel (d) shows the formation height of the line profile minimum $h_{\lambda_{\mathrm{min}}}=h(\tau_{\lambda,\mathrm{min}}=1)$, panel (e) shows the vertical velocity at $h_{\lambda_{\mathrm{min}}}$, and panel (f) shows the vertical component of the magnetic field at $h_{\lambda_{\mathrm{min}}}$. The line profile minimum and the corresponding formation height are individually determined for each column in the atmosphere. The color scale limits are clipped to increase the contrast of the images (see text). The intensity (panels a and b), as well as the formation heights that were used to create panels (c, d, e, and f), originate from the im rt computation from the snapshot muram_en_518000_503s.
• Figure 2: Comparison of spatially averaged profiles (panels a and b) and corresponding bisectors (panels c, d, and e) of the Caii $\lambda854.2$ nm line. The black curves show spatially averaged qs spectra from the ftsatlas. The red curve shows a time average of the full isotope computation of four snapshots separated by $2\,\mathrm{min}$ (see Appendix \ref{['app:time-variation']} for more details) together with the standard deviation multiplied by a factor of eight in light red color (not visible in panel a but in panel b). The orange, green, and blue curves show data from the single snapshot muram_en_518000_503s. In orange, we show the spatially averaged synthetic spectra computed by taking all isotopes of calcium into account. The other curves show similar profiles but computed without isotopic splitting (green), and using a composite model atom (blue). The wavelength axis of the average profiles is centered at $\lambda - \lambda_{\mathrm{^{40}Ca,rest}}$ where $\lambda_{\mathrm{^{40}Ca,rest}}$ is the rest wavelength of $^{40}\mathrm{Ca}$, the most abundant calcium isotope. The bisectors are presented in three different ways. Once they are all centered on the wavelength of the corresponding line profile minimum (panel c), once the bisectors are shown on an absolute wavelength scale (panel d), and once the bisectors are centered on the wavelength where the bisector intensity is $60\%$ of the continuum intensity (panel e).
• Figure 3: Profiles and bisectors averaged over pixels with similar atmospheric properties. We show data from synthetic spectra once taking all isotopes into account (orange) and once taking only the most abundant isotope (green) into account. The top row (panels a, b, and c) shows results computed from a selection of strong (solid lines) and weak (dashed lines) magnetic fields in the formation height region of the line core (see text for details). The bottom row (panels d, e, and f) shows only profiles and bisectors over the pixels harbouring upflows (solid lines) or downflows (dashed lines) in the formation height region of the line core (see text). The bisectors are shown once on an absolute wavelength scale (panels b and e) and on a scale where the bisectors are centered on the line profile minimum wavelength $\lambda_{\mathrm{min}}$. The synthetic data corresponds to snapshot muram_en_518000_503s.
• Figure 4: Time variation of spatially averaged line profile in the simulation. We show the spatially averaged line profiles (panel a) and their bisectors (panel b) from four snapshots of the simulation that are averaged by approximately $2 \min$ of simulation time. The snapshot at $t=2.06 \min$ (blue curves) corresponds to the snapshot muram_en_518000_503s that was discussed in detail in the main text. The line profiles are shown in a large wavelength window to show that there is also a time variation in the far wings of the spectral line in the simulation. The bisectors in panel (b) are shown on an absolute wavelength scale, similar to Fig. \ref{['fig:fig1_av_spectra_and_bisector']} (d). All the synthetic profiles in this figure were computed including isotopes and thus correspond to im computations.
• Figure 5: Determination of line width. The line width is measured at half the intensity between the wing intensity $I_{\mathrm{wing}}$ and the profile minimum intensity $I_{\mathrm{min}}$. The intensity $I_{\mathrm{wing}}$ is taken as the average intensity at $\pm 0.6 \AA$ away from the line profile minimum. The obtained line widths for the different computations and the observation are indicated in the figure.