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

Abstract

Context. Thermal non-equilibrium (TNE) is a phenomenon that can occur in solar coronal loops when the heating is quasi-constant and highly-stratified. Under such heating conditions, coronal loops undergo cycles of evaporation and condensation. The recent observations of ubiquitous long-period intensity pulsations in coronal loops and their relationship with coronal rain have demonstrated that understanding the characteristics of TNE cycles is an essential step in constraining the circulation of mass and energy in the corona. Aims. We report unique observations with the Solar Dynamics Observatory (SDO) and the Swedish 1-m Solar Telescope (SST) that link the captured thermal properties across the extreme spatiotemporal scales covered by TNE processes. Methods. Within the same coronal loop bundle, we captured 6 hr period coronal intensity pulsations in SDO/AIA and coronal rain observed off-limb in the chromospheric Hα and Ca II K spectral lines with SST/CRISP and SST/CHROMIS. We combined a multithermal analysis of the cycles with AIA and an extensive spectral characterisation of the rain clumps with the SST. Results. We find clear evidence of evaporation-condensation cycles in the corona which are linked with periodic coronal rain showers. The high-resolution spectroscopic instruments at the SST reveal the fine-structured rain strands and allow us to probe the cooling phase of one of the cycles down to chromospheric temperatures. Conclusions. These observations reinforce the link between long-period intensity pulsations and coronal rain. They also demonstrate the capability of TNE to shape the dynamics of active regions on the large scales as well as on the smallest scales currently resolvable.

Magnetic field inference in active region coronal loops using coronal rain clumps

“…The repeating cycle corresponds to periods of evaporation of chromospheric material that accumulates within the loop and periods of drainage of part of the material inside the loop in the form of precipitation that falls along the magnetic field lines. These condensation-evaporation cycles have their extreme ultraviolet (EUV) observational counterparts in the long period EUV pulsations, which have been described in recent studies (see Pelouze et al 2020;Auchère et al 2016Auchère et al , 2018Froment et al 2020).…”

Temporal evolution of oscillating coronal loops

“…This is in contrast to long-lived loop systems, which may be analysed for tens of hours (e.g. Auchère et al 2016;Froment et al 2020).…”

“…Such individual loop threads are expected to be nonequilibrium structures, with lifetimes from tens of minutes to several hours predicted by thermodynamic analysis. In whole loop bundles, observation of thermal cycles are common (see Froment et al 2020, and references therein), with periods from 2-16 h. Additionally, various plasma flows can be detected in a loop, which may result in a constant evolution. Su et al (2018) reported density variations of the oscillating loop and related them to the period variation of the kink mode.…”

Transition region adaptive conduction (TRAC) in multidimensional magnetohydrodynamic simulations

“…One topic intimately related to the evaporation process that the TRAC approach wishes to improve is the chromospheric evaporation-coronal condensation scenario (Antiochos et al 2000). 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).…”