Constraints on Acoustic Wave Energy Fluxes and Radiative Losses in the Solar Chromosphere from Non-LTE Inversions

TL;DR

This study uses non-LTE inversions of spectroscopic data and simulations to evaluate the energy balance in the solar chromosphere, revealing that acoustic fluxes are generally insufficient to offset radiative losses, especially in higher layers.

Contribution

It provides a comprehensive analysis combining observations and simulations to assess acoustic fluxes and radiative losses, highlighting potential underestimations by inversion methods.

Findings

Inversions may underestimate acoustic fluxes across chromospheric heights.

Radiative losses are slightly overestimated by the inversion approach.

The ratio of acoustic flux to radiative losses is below unity in most regions, especially higher in the chromosphere.

Abstract

Accurately assessing the balance between acoustic wave energy fluxes and radiative losses is critical for understanding how the solar chromosphere is thermally regulated. We investigate the energy balance in the chromosphere by comparing deposited acoustic flux and radiative losses under quiet and active solar conditions using non-local thermodynamic equilibrium (NLTE) inversions with the Stockholm Inversion Code (STiC). To achieve this, we utilize spectroscopic observations from the Interferometric BIdimensional Spectrometer (IBIS) in the Na I 5896 \r{A} and Ca II 8542 \r{A} lines and from the Interface Region Imaging Spectrograph (IRIS) in the Mg II h and k lines to self-consistently derive spatially resolved velocity power spectra and cooling rates across different heights in the atmosphere. Additionally, we use snapshots of a three-dimensional radiative-magnetohydrodynamics simulation to investigate the systematic effects of the inversion approach, particularly the attenuation effect on the velocity power spectra and the determination of the cooling rates. The results indicate that inversions potentially underestimate acoustic fluxes at all chromospheric heights while slightly overestimating the radiative losses when fitting these spectral lines. However, even after accounting for these biases, the ratio of acoustic flux to radiative losses remains below unity in most observed regions, particularly in the higher layers of the chromosphere. We also observe a correlation between the magnetic field inclination in the photosphere and radiative losses in the low chromosphere in plage, which is evidence that the field topology plays a role in the chromospheric losses.