Temperature constraints from inversions of synthetic solar optical, UV and radio spectra

TL;DR

This study assesses how millimeter-wavelength observations, especially from ALMA, can enhance the accuracy of temperature inversions in the solar chromosphere when combined with optical and UV diagnostics.

Contribution

It demonstrates that including mm-continuum data from ALMA improves temperature retrievals in the upper chromosphere, highlighting the value of multi-wavelength inversion approaches.

Findings

Including mm-continuum improves temperature inference at certain optical depths.

ALMA bands enhance chromospheric temperature accuracy, especially at higher layers.

Simultaneous optical, UV, and mm inversions provide strong constraints without ALMA, but ALMA still adds value.

Abstract

High-resolution observations of the solar chromosphere at millimeter wavelengths are now possible with the Atacama Large Millimeter Array (ALMA), promising to tackle many open problems in solar physics. Observations from other ground and space-based telescopes will greatly benefit from coordinated endeavors with ALMA, yet the diagnostic potential of combined optical, ultraviolet and mm observations has remained mostly unassessed. In this paper we investigate whether mm-wavelengths could aid current inversion schemes to retrieve a more accurate representation of the temperature structure of the solar atmosphere. We performed several non-LTE inversion experiments of the emergent spectra from a snapshot of 3D radiation-MHD simulation. We included common line diagnostics such as CaII H, K, 8542 \AA$\,$ and MgII h and k, taking into account partial frequency redistribution effects, along with the continuum around 1.2 mm and 3 mm. We found that including the mm-continuum in inversions allows a more accurate inference of temperature as function of optical depth. The addition of ALMA bands to other diagnostics should improve the accuracy of the inferred chromospheric temperatures between $\log \tau\sim[-6,-4.5]$ where the CaII and MgII lines are weakly coupled to the local conditions. However, we found that simultaneous multi-atom, non-LTE inversions of optical and UV lines present equally strong constraints in the lower chromosphere and thus are not greatly improved by the 1.2 mm band. Nonetheless, the 3 mm band is still needed to better constrain the mid-upper chromosphere.