Thermodynamic fluctuations in solar photospheric three-dimensional convection simulations and observations

TL;DR

This study validates 3D solar convection simulations by comparing synthetic spectra with high-resolution observations, finding no excessive thermodynamic fluctuations at resolutions up to 0.3".

Contribution

It demonstrates that current 3D radiative MHD simulations accurately reproduce observed spectral line properties without excessive thermodynamic fluctuations.

Findings

Most line parameters match well between observations and degraded simulations.

Inversion results show low macroturbulent velocities at full resolution.

Temperature fluctuations in simulations are consistent with observational data.

Abstract

Numerical 3D radiative (M)HD simulations of solar convection are used to understand the physical properties of the solar photosphere. To validate this approach, it is important to check that no excessive thermodynamic fluctuations arise as a consequence of the partially incomplete treatment of radiative transfer. We investigate the realism of 3D convection simulations carried out with the Stagger code. We compared the characteristic properties of several spectral lines in solar disc centre observations with spectra synthesized from the simulations. We degraded the synthetic spectra to the spatial resolution of the observations using the continuum intensity distribution. We estimated the necessary spectral degradation by comparing atlas spectra with averaged observed spectra. In addition to deriving a set of line parameters directly, we used the SIR code to invert the spectra. Most of the line parameters from the observational data are matched well by the degraded simulation spectra. The inversions predict a macroturbulent velocity below 10 m/s for the simulation at full spatial resolution, whereas they yield ~< 1000 m/s at a spatial resolution of 0.3". The temperature fluctuations in the inversion of the degraded simulation do not exceed those from the observational data (of the order of 100-200 K rms for -2<log tau<-0.5). The comparison of line parameters in spatially averaged profiles with the averaged values of line parameters in spatially resolved profiles indicates a significant change of (average) line properties at a spatial scale between 0.13" and 0.3". Up to a spatial resolution of 0.3", we find no indications of the presence of excessive thermodynamic fluctuations in the 3D HD simulation. To definitely confirm that simulations without spatial degradation contain fully realistic thermodynamic fluctuations requires observations at even better spatial resolution.