Thermal instabilities: Fragmentation and field misalignment of filament fine structure

Abstract

Context. Prominences show a surprising amount of fine structure and it is widely believed that their threads, as seen in Hα observations, provide indirect information concerning magnetic field topology. Both prominence and coronal rain condensations most likely originate from thermal instabilities in the solar corona. It is still not understood how non-linear instability evolution shapes the observed fine structure of prominences. Investigating this requires multidimensional, high-resolution simulations to resolve all emerging substructure in great detail. Aims. We investigate the spontaneous emergence and evolution of fine structure in high-density condensations formed through the process of thermal instability under typical solar coronal conditions. Our study reveals intricate multidimensional processes that occur through in situ condensations in a representative coronal volume in a low plasma beta regime. Methods. We quantified slow wave eigenfunctions used as perturbations and discuss under which conditions the thermal mode is unstable when anisotropic thermal conduction effects are included. We performed 2D and 3D numerical simulations of interacting slow magnetohydrodynamic (MHD) wave modes when all relevant non-adiabatic effects are included. Multiple levels of adaptive mesh refinement achieve a high resolution near regions with high density, thereby resolving any emerging fine structure automatically. Our study employs a local periodic coronal region traversed by damped slow waves inspired by the presence of such waves observed in actual coronal magnetic structures. Results. We show that the interaction of multiple slow MHD wave modes in a regime unstable to the thermal mode leads to thermal instability. This initially forms pancake-like structures almost orthogonal to the local magnetic field, while low-pressure induced inflows of matter generate rebound shocks. This is succeeded by the rapid disruption of these pancake sheets through thin-shell instabilities evolving naturally from minute ram pressure imbalances. This eventually creates high-density blobs accompanied by thread-like features from shear flow effects. The further evolution of the blobs follows the magnetic field lines, such that a dynamical realignment with the background magnetic field appears. However, the emerging thread-like features are not at all field-aligned, implying only a very weak link between fine structure orientation and magnetic field topology. Conclusions. As seen in our synthetic Hα views, threads formed by non-linear thermal instability evolution do not strictly outline magnetic field structure and this finding has far-reaching implications for field topology interpretations based on Hα observations.

Prominence formation by levitation-condensation at extreme resolutions

“…In contrast to these works, we will here focus on a 'levitation-condensation' configuration, where thermal nonequilibrium cycles are completely absent, since there will be no connectivity to lower layers, and everything is happening due to Thermal Instability and other magnetohydrodynamic processes. It should be noted that the pure Thermal Instability mechanism has also been studied in more idealized settings by Claes & Keppens (2019); Claes et al (2020), pointing out that the initial stages of the Thermal Instability process do not lead to a perfect alignment between condensations and magnetic field lines, bringing further complications to interpretations of observed fine-structure.…”

When hot meets cold: post-flare coronal rain

“…Indeed, the temperature of bright loops in this 17.1 passband (about 10 5.8 K) is not far from the critical temperature for the onset of catastrophic cooling (this is density and temperature dependent, but generally happens below 2 MK according to [41]). Condensations can be triggered by thermal instability near bright loops and these suddenly formed structures grow fastest across magnetic field lines (counterintuitive due to the field-aligned thermal conduction, but see [22,29,30,42]).…”

“…Coronal rain is also observed in non-flaring coronal loops, is frequently found in loops of active regions [15,16,17,18,19,20,21], and this type of coronal rain has been studied previously using magnetohydrodynamic (MHD) simulations [22,23,24,25,26]. It has been generally accepted that these rain blobs are generated in a catastrophic cooling process, essentially caused by thermal instability [27,28,29,30]. In a catastrophic cooling event, local temperatures drop from 1 MK to below 0.1 MK within one minute, while local densities can increase by orders of magnitude [14].…”

Transition region adaptive conduction (TRAC) in multidimensional magnetohydrodynamic simulations

“…In solar physics, the transition region (TR), a thin layer between the cool chromosphere and the hot corona, might be the most concerned. Similarly, whenever a coronal condensation forms by means of thermal instability (e.g., Fang et al 2015;Xia & Keppens 2016;Claes et al 2020), the dynamically created boundary between the prominence or coronal rain blob and the corona is yet another example of a TR.…”

“…The evaporation will trigger thermal nonequilibrium (TNE) in the magnetic loop, which can explain the omnipresent coronal rain or prominence formation (Klimchuk 2019;Froment et al 2020;Antolin 2020). Succesful condensations also involve thermal instability (TI), a well-known linear instability (Parker 1953;Field 1965) that affects the otherwise marginal entropy mode (Claes & Keppens 2019;Claes et al 2020). In the evaporation-condensation scenario, a physically correct evaporation is crucial to either TNE cycles and/or to encounter conditions ripe for a condensation into solar prominence or rain due to in situ linear TI.…”