Observational signatures of mixing-induced cooling in the Kelvin-Helmholtz instability

TL;DR

This study uses 2D simulations to show that Kelvin-Helmholtz mixing in the solar corona leads to cooling signatures, with increased emission in warm lines and decreased emission in cooler lines, indicating mixing-induced cooling rather than heating.

Contribution

It provides the first detailed observational signature analysis of cooling in Kelvin-Helmholtz mixing layers in the solar corona using synthetic emission modeling.

Findings

Cooling dominates over turbulent heating in the mixing layer.

Enhanced emission in warm spectral lines occurs during cooling.

Decreased emission in cooler lines can indicate mixing-induced cooling.

Abstract

Cool ($\approx 10^4$K), dense material permeates the hot ($\approx 10^6$K), tenuous solar corona in form of coronal condensations, for example prominences and coronal rain. As the solar atmosphere evolves, turbulence can drive mixing between the condensations and the surrounding corona, with the mixing layer exhibiting an enhancement in emission from intermediate temperature ($\approx10^5$K) spectral lines, which is often attributed to turbulent heating within the mixing layer. However, radiative cooling is highly efficient at intermediate temperatures and numerical simulations have shown that radiative cooling can far exceed turbulent heating in prominence-corona mixing scenarios. As such the mixing layer can have a net loss of thermal energy, i.e., the mixing layer is cooling rather than heating. Here, we investigate the observational signatures of cooling processes in Kelvin-Helmholtz mixing between a prominence thread and the surrounding solar corona through 2D numerical simulations. Optically thin emission is synthesised for Si IV, along with optically thick emission for H$\alpha$, Ca II K and Mg II h using Lightweaver The Mg II h probes the turbulent mixing layer, whereas H$\alpha$ and Ca II K form within the thread and along its boundary respectively. As the mixing evolves, intermediate temperatures form leading to an increase in Si IV emission, which coincides with increased radiative losses. The simulation is dominated by cooling in the mixing layer, rather than turbulent heating, and yet enhanced emission in warm lines is produced. As such, an observational signature of decreased emission in cooler lines and increased emission in hotter lines may be a signature of mixing, rather than an implication of heating.