Anomalous emission from Li- and Na-like ions in the Corona heated via Alfv\'{e}n wave

TL;DR

This study investigates how Alfvén wave-driven dynamics in the solar corona cause non-equilibrium ionization effects in Li- and Na-like ions, leading to anomalous ultraviolet emission intensities that deviate from equilibrium expectations.

Contribution

It introduces a numerical model coupling NEI with MHD equations to analyze ionization state variations during Alfvén wave heating in coronal loops, highlighting the impact of evaporation and condensation phases.

Findings

Ionization fractions of Li- and Na-like ions increase during evaporation, causing up to 1.6 times intensity enhancement.

Shock interactions cause temporary deviations from ionization equilibrium, but these are negligible over time.

Over/under-fractionation depends on mass circulation, affecting emission intensities during different phases.

Abstract

The solar ultraviolet intensities of spectral lines originating from Li- and Na-like ions have been observed to surpass the expectations derived from plasmas with coronal approximation. The violation of the coronal approximation can be partially attributed to non-equilibrium ionization (NEI) due to dynamic processes occurring in the vicinity of the transition region. To investigate the impact of these dynamics in Alfv\'{e}n-wave-heated coronal loop, a set of equations governing NEI for multiple ion species was solved numerically in conjunction with 1.5-dimensional magnetohydrodynamic equations. Following the injection of Alfv\'{e}n waves from the photosphere, the system undergoes a time evolution characterized by phases of evaporation, condensation, and quasi-steady states. During the evaporation phase, the ionization fractions of Li- and Na-like ions were observed to increase when compared to the fractions in ionization equilibrium, which lead to the intensity enhancement of up to 1.6. This over-fractionation of Li- and Na-like ions was found to be induced by the evaporation process. While collisions between shocks and the transition region temporarily led to deviations from ionization equilibrium, on average over time, these deviations were negligible. Conversely, under-fractions of the ionization fraction led to intensity reduction of down to 0.9 during the condensation phase and the quasi-steady state. Given the dependency of the over/under-fractionation on mass circulations between the chromosphere and the corona, these observations will serve as valuable benchmarks to validate not only Alfv\'{e}n wave models but also other existing mechanisms on coronal heating.