Spectral Gradient of the Thermal Millimetre Continuum as a Diagnostic for Optical Thickness in the Solar Atmosphere

TL;DR

This paper demonstrates how the spectral gradient of the thermal millimetre continuum can diagnose the optical thickness regime of the solar atmosphere, aiding interpretation of solar observations.

Contribution

It provides a theoretical framework and empirical validation for using the millimetre spectral gradient as a diagnostic tool for optical thickness in the solar atmosphere.

Findings

Logarithmic spectral gradient effectively indicates optical thickness regimes.

Correction for non-constant Gaunt factor improves diagnostic accuracy.

Spectral gradient distinguishes between optically thin, transitional, and thick regimes.

Abstract

Aims. In this article we aim to show how the gradient of the thermal millimetre continuum spectrum, as emitted from the quiet solar atmosphere, may be used as a diagnostic for the optical thickness regime at the centre of the observing frequency band. Methods. We show the theoretical derivation of the gradient of the millimetre continuum for both logarithmic- and linear-scale spectra. We compare this expression with the empirical relationship between the slope of the millimetre continuum spectrum and the plasma optical thickness computed from both isothermal and multi-thermal two-dimensional cylindrical radiative transfer models. Results. It is found that the logarithmic-scale spectral gradient provides a clear diagnostic for the optical thickness regime for both isothermal and multi-thermal plasmas, provided that a suitable correction is made for a non-constant gaunt factor over the frequency band. For the use of observers we present values for this correction at all ALMA bands and at a wide range of electron temperatures. Conclusions. We find that the spectral gradient can be used to find (a) whether the source is fully optically thin, (b) the optical thickness of the source if it lies within the transitional regime between optically thin and thick plasma ({\tau} $\approx$ $10^{-1} - 10^{1}$), or (c) whether the source is fully optically thick for an isothermal plasma. A multi-thermal plasma will act the same as an isothermal plasma for case (a), however, the transitional regime will only extend from {\tau} $\approx$ $10^{-1} - 10^{0}$ . Above {\tau} = 1 the slope of the continuum will depend increasingly on the temperature gradient, as well as the optical thickness, reducing the reliability of the diagnostic.