Inter-network regions of the Sun at millimetre wavelengths

TL;DR

This study uses 3D MHD simulations to analyze how millimetre wavelength observations can probe the solar chromosphere's thermal structure and dynamics, highlighting the effects of wavelength and disk position on brightness temperature.

Contribution

It provides synthetic brightness temperature maps at various wavelengths based on advanced simulations, improving understanding of the chromosphere's structure and the impact of ionisation equilibrium deviations.

Findings

Brightness temperature increases with wavelength and towards the limb.

Formation height varies with wavelength and disk position, affecting observed intensity.

Intensity contribution functions can be broad or double-peaked, indicating complex formation processes.

Abstract

The continuum intensity at wavelengths around 1 mm provides an excellent way to probe the solar chromosphere. Future high-resolution millimetre arrays, such as the Atacama Large Millimeter Array (ALMA), will thus produce valuable input for the ongoing controversy on the thermal structure and the dynamics of this layer. Synthetic brightness temperature maps are calculated on basis of three-dimensional radiation (magneto-)hydrodynamic (MHD) simulations. While the millimetre continuum at 0.3mm originates mainly from the upper photosphere, the longer wavelengths considered here map the low and middle chromosphere. The effective formation height increases generally with wavelength and also from disk-centre towards the solar limb. The average intensity contribution functions are usually rather broad and in some cases they are even double-peaked as there are contributions from hot shock waves and cool post-shock regions in the model chromosphere. Taking into account the deviations from ionisation equilibrium for hydrogen gives a less strong variation of the electron density and with it of the optical depth. The result is a narrower formation height range. The average brightness temperature increases with wavelength and towards the limb. The relative contrast depends on wavelength in the same way as the average intensity but decreases towards the limb. The dependence of the brightness temperature distribution on wavelength and disk-position can be explained with the differences in formation height and the variation of temperature fluctuations with height in the model atmospheres.