# Non-Equilibrium Processes in the Solar Corona, Transition Region, Flares, and Solar Wind \textit{(Invited Review)}

## TL;DR

This review discusses non-equilibrium processes like non-Maxwellian distributions and ionization in the solar atmosphere, highlighting observational evidence, modeling approaches, and future instrumentation for detection.

## Contribution

It provides a comprehensive overview of non-equilibrium phenomena in the solar environment, integrating observational constraints, modeling techniques, and instrumentation prospects.

## Key findings

- Detection of non-Maxwellian distributions in the solar corona and wind
- Signatures of non-equilibrium ionization predicted for various plasma conditions
- Future instruments can improve detection of non-equilibrium processes

## Abstract

We review the presence and signatures of the non-equilibrium processes, both non-Maxwellian distributions and non-equilibrium ionization, in the solar transition region, corona, solar wind, and flares. Basic properties of the non-Maxwellian distributions are described together with their influence on the heat flux as well as on the rates of individual collisional processes and the resulting optically thin synthetic spectra. Constraints on the presence of high-energy electrons from observations are reviewed, including positive detection of non-Maxwellian distributions in the solar corona, transition region, flares, and wind. Occurrence of non-equilibrium ionization is reviewed as well, especially in connection to hydrodynamic and generalized collisional-radiative modelling. Predicted spectroscopic signatures of non-equilibrium ionization depending on the assumed plasma conditions are summarized. Finally, we discuss the future remote-sensing instrumentation that can be used for detection of these non-equilibrium phenomena in various spectral ranges.