On the Occurrence of Thermal Nonequilibrium in Coronal Loops

Abstract

Long-period EUV pulsations, recently discovered to be common in active regions, are understood to be the coronal manifestation of thermal nonequilibrium (TNE). The active regions previously studied with EIT/Solar and Heliospheric Observatory and AIA/SDO indicated that long-period intensity pulsations are localized in only one or two loop bundles. The basic idea of this study is to understand why. For this purpose, we tested the response of different loop systems, using different magnetic configurations, to different stratifications and strengths of the heating. We present an extensive parameter-space study using 1D hydrodynamic simulations (1020 in total) and conclude that the occurrence of TNE requires specific combinations of parameters. Our study shows that the TNE cycles are confined to specific ranges in parameter space. This naturally explains why only some loops undergo constant periodic pulsations over several days: since the loop geometry and the heating properties generally vary from one loop to another in an active region, only the ones in which these parameters are compatible exhibit TNE cycles. Furthermore, these parameters (heating and geometry) are likely to vary significantly over the duration of a cycle, which potentially limits the possibilities of periodic behavior. This study also confirms that long-period intensity pulsations and coronal rain are two aspects of the same phenomenon: both phenomena can occur for similar heating conditions and can appear simultaneously in the simulations.

Multi-scale observations of thermal non-equilibrium cycles in coronal loops

“…We can speculate that to achieve such fast cycles for a loop bundle as long as the one presented in Auchère et al (2018), the heating strength was probably very different than for the active region presently studied. This is consistent with the simulations of Froment et al (2018).…”

“…Some other loops did not seem to show any signs of TNE (no pulsations and no rain). However, stratified and high frequency heating can be present beyond structures that are experiencing TNE, as shown in Froment et al (2018). In some cases, close to TNE-prone conditions, the loops global behaviour can be dominated by siphon flows without thermal cycles ( also see Klimchuk & Luna 2019).…”

Spectroscopic detection of coronal plasma flows in loops undergoing thermal non-equilibrium cycles

“…Enhanced emission in the coronal channels of EIT or AIA occurs during cycles in which the plasma reaches a peak temperature of a few million degrees. This behavior is well reproduced in one-dimensional hydrodynamic simulations which compute the response of the plasma in a loop to a given heating (Kuin & Martens 1982;Martens & Kuin 1983;Karpen et al 2001;Müller et al 2003Müller et al , 2004Müller et al , 2005Karpen et al 2005;Antolin et al 2010;Xia et al 2011;Mikić et al 2013;Mok et al 2016;Froment et al 2017Froment et al , 2018. In particular, Froment et al (2017) were able to convincingly reproduce the intensity and emission measure from one of the events observed with AIA presented in their previous paper (Froment et al 2015).…”

“…These pulsations have been interpreted as resulting from Thermal Non-Equilibrium (TNE) (Auchère et al 2014;Froment et al 2015;Auchère et al 2016;Froment et al 2017Froment et al , 2018, which can result from of a quasi-constant heating localized near the loops footpoints. In this case, there may exist no equilibrium between the heating near the footpoints and the radiative Present address: Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B bus 2400, 3001 Leuven, Belgium. losses in the corona (Antiochos & Klimchuk 1991;Antiochos et al 1999;Antiochos et al 2000;Karpen et al 2001;Klimchuk & Luna 2019;Antolin 2019;Klimchuk 2019).…”

The effects of numerical resolution, heating timescales and background heating on thermal non-equilibrium in coronal loops

“…We model a coronal loop of total length 180 Mm with a small chromosphere attached to each end and select the largest grid cells in our calculations to have a width of 1 Mm. Thus, at the highest refinement level, the minimum cell width is 122 m. 2010; Mikić et al 2013;Froment et al 2018), we first consider the case of steady footpoint heating where the spatial profile is given by the sum of two Gaussian peaks (one at each loop leg), each defined as,…”