Mixing-induced thermal instabilities and coronal condensations

TL;DR

This paper shows that mixing-induced cooling in a fully 3D radiative MHD Kelvin-Helmholtz setup can spontaneously generate thermally unstable regions within a condensation–corona mixing layer. Radiative losses, modeled as $\rho^2 \Lambda(T)$ with a Chianti-based curve cut off above $10^6$ K, drive cooling in the intermediate-temperature mixing layer, producing long, narrow condensations oriented perpendicular to the magnetic field and continually replenishing cool mass. The thermal instabilities contribute about $15$–$20\%$ of the total radiative losses, with a peak around $t \approx 170$, demonstrating a self-consistent mechanism for coronal condensations akin to prominences and coronal rain. The results highlight the importance of 3D dynamics for condensation formation and suggest turbulent conduction and shear-driven elongation play key roles in setting the observed filament scales in the solar atmosphere.

Abstract

Cool, dense material is frequently observed to permeate the hot, tenuous solar corona in the form of prominences, spicules and coronal rain. Both the cool material and surrounding corona exist at temperatures that are effectively thermally stable, in that their local radiative losses occur on relatively long timescales compared to the dynamics. However, as the solar atmosphere evolves, driving mixing between the condensations and surrounding hot material, intermediate temperatures form, which can become subject to highly efficient radiative losses. The thermal energy lost due to radiation can far exceed the turbulent heating thus the system undergoes mixing-induced cooling. Here, a 3D radiative MHD simulation is performed of the shear-driven Kelvin-Helmholtz Instability (KHI) occurring between a cool condensation and the hot solar corona. During the evolution, thermal instabilities form naturally within the mixing layer, and grow with time to produce long, narrow structures that extend perpendicular to the magnetic field. The thermal instabilities form self-consistently within the mixing layer as small isolated events, and are then stretched by the background flows to create long structures in relatively narrow planes. The turbulent flows agitate the condensations and cause them to fragment, creating smaller localised clumps of cool, dense (prominence-like) material that can merge and further fragment. In the presented simulation, the thermal instabilities act to replenish the cool, dense material lost due to mixing, with the total mass of cool material being approximately constant through time. By analysing the thermal energy loss due to optically-thin radiation, thermal instabilities are found to account for 15-20\% of all radiative losses in the turbulent plasma.

Figures (13)

• Figure 1: Normalised loss function $\Lambda (\hat{T}T)$. The blue line shows the default Chianti generated loss curve with the default abundance file. The orange line shows the modified loss curve used in this work which has zero losses above $\hat{T}T=10^6$K.
• Figure 2: Slice of the simulation at $z=0$ at different times showing the density (left), temperature (centre) and losses (right).
• Figure 3: 3D volume plots at time $t=336$ of density (a), temperature (b), losses (c), and thermal instability-like structures (d). Note that the opacity of the values outside of the mixing layer have been set to zero.
• Figure 4: Slices of the density in the 3D simulation at time $t=236$. The thermal instabilities are overplotted. Note that the thermal instabilities coincide with high density, low temperature regions.
• Figure 5: Properties of an extracted thermal instability at $y=-0.436$ showing the temperature (a), density (b), losses $\Lambda (T)$ (c), and pressure (d). The black fieldlines show the magnetic field. The green arrows show the perturbation velocity field in the plane ($\textbf{v}-\bar{\textbf{v}}$)