Thermally enhanced tearing in solar current sheets: Explosive reconnection with plasmoid-trapped condensations

Abstract

Context. Thermal instability plays a major role in condensation phenomena in the solar corona, e.g. for coronal rain and prominence formation. In flare-relevant current sheets, tearing instability may trigger explosive reconnection and plasmoid formation. However, how both instabilities influence the disruption of current concentrations in the solar corona has received less attention to date. Aims. We explore how the thermal and tearing modes reinforce each other in the fragmentation of a current sheet in the solar corona through an explosive reconnection process, characterized by the formation of plasmoids which interact and trap condensing plasma. Methods. We use a resistive magnetohydrodynamic (MHD) simulation of a 2D current layer, incorporating the non-adiabatic effects of optically thin radiative energy loss and background heating using the open-source code MPI-AMRVAC. Multiple levels of adaptive mesh refined grids are used for achieving a high resolution to resolve the fine structures during the evolution of the system. Results. Our parametric survey explores different resistivities and plasma-β to quantify the instability growth rate in the linear and nonlinear regimes. We notice that for dimensionless resistivity values within 10 −4 − 5 × 10 −3 , we get explosive behavior where thermal instability and tearing behavior reinforce each other. This is clearly below the usual critical Lundquist number range of pure resistive explosive plasmoid formation. We calculate the mean growth rate for the linear phase and different non-linear phases of the evolution. The non-linear growth rates follow weak power-law dependency with resistivity. The fragmentation of the current sheet and the formation of the plasmoids in the nonlinear phase of the evolution due to the thermal and tearing instabilities are obtained. The formation of plasmoids is noticed for the Lundquist number (S L ) range 4.6 × 10 3 − 2.34 × 10 5 . We quantify the temporal variation of the plasmoid numbers and the density filling factor of the plasmoids for different physical conditions. We also find that the maximum plasmoid numbers scale as S 0.223 L .Within the nonlinearly coalescing plasmoid chains, localized cool condensations gather, realizing density and temperature contrasts similar to coronal rain or prominences.

2.5D Magnetohydrodynamic Simulation of the Formation and Evolution of Plasmoids in Coronal Current Sheets

“…Sen & Keppens (2022) used a similar dimensionless resistivity in their numerical simulation of coronal reconnection. It is also similar to those used in the study of plasmoid instabilities by various authors (e.g., Bhattacharjee et al 2009;Huang & Bhattacharjee 2010, 2012, 2013Huang et al 2015;Sen & Keppens 2022).…”

“…Now for Lundquist numbers 2.51 × 10 5 and 6.28 × 10 5 , Bhattacharjee et al (2009Bhattacharjee et al ( ) used [2001Bhattacharjee et al ( , 1001Bhattacharjee et al ( ] and [3001, 1001 grid points, respectively. Also, Sen & Keppens (2022) have studied plasmoid-related physics with [2048,2048] grid points with the smallest grid size being 125 km using MPI-AMRVAC and the highest value of their Lundquist number is 2.34 × 10 5 . Thus, our value of the Lundquist number, i.e., 4.8 × 10 5 is about twice their value.…”

Collisional ionisation, recombination, and ionisation potential in two-fluid slow-mode shocks: Analytical and numerical results

“…The thermal instability occurs when the radiative losses cause a reduction in the pressure, driving an inflow which enhances the temperature, allowing for additional cooling. The thermal instability is critical for the formation of cool condensations in the solar atmosphere 30,31 , planetary nebulae 32 , and the intergalactic medium 33 . It is possible that non-steady cooling shocks could result in a post-shock condition that is unstable to the thermal instability, potentially enhancing the formations of condensations.…”

A comparative study of resistivity models for simulations of magnetic reconnection in the solar atmosphere

“…For plasmoid-mediated reconnection in an adiabatic medium, the number of plasmoids has been analytically predicted to scale with the Lundquist number as S 0.375 L (Loureiro et al 2007). For the non-adiabatic case, Sen & Keppens (2022) numerically found the maximum plasmoid number in a 2D Harris current sheet to scale as S 0.223 L . In both cases, the number of plasmoids increases slowly with the Lundquist number.…”