The Chromospheric Solar Limb Brightening at Radio, Millimeter, Sub-millimeter, and Infrared Wavelengths

TL;DR

This study evaluates semi-empirical chromospheric models against high-resolution observations of solar limb brightening across radio to infrared wavelengths, revealing significant discrepancies and emphasizing the importance of high chromospheric structures.

Contribution

It systematically compares VALC and C7 models with observations, highlighting their limitations in reproducing solar radii and limb brightening at various wavelengths.

Findings

Discrepancies of up to 12,000 km between models and observations at 20 GHz.

Models fail to accurately reproduce solar radii at radio to infrared wavelengths.

High chromospheric structures better explain observed limb brightening.

Abstract

Observations of the emission at radio, millimeter, sub-millimeter, and infrared wavelengths in the center of the solar disk validate the auto-consistence of semi-empirical models of the chromosphere. Theoretically, these models must reproduce the emission at the solar limb. In this work, we tested both the VALC and the C7 semi-empirical models by computing their emission spectrum in the frequency range from 2 GHz to 10 THz, at solar limb altitudes. We calculate the Sun's theoretical radii as well as their limb brightening. Non-Local Thermodynamic Equilibrium (NLTE) was computed for hydrogen, electron density, and H-. In order to solve the radiative transfer equation a 3D geometry was employed to determine the ray paths and Bremsstrahlung, H-, and inverse Bremsstrahlung opacity sources were integrated in the optical depth. We compared the computed solar radii with high resolution observations at the limb obtained by Clark (1994). We found that there are differences between observed and computed solar radii of 12000 km at 20 GHz, 5000 km at 100 GHz, and 1000 km at 3 THz for both semi-empirical models. A difference of 8000 km in the solar radii was found comparing our results against heights obtained from H{\alpha} observations of spicules-off at the solar limb. We conclude that the solar radii can not be reproduced by VALC and C7 semi-empirical models at radio - infrared wavelengths. Therefore, the structures in the high chromosphere provides a better measurement of the solar radii and their limb brightening as shown in previous investigations.