Evidence of thermal conduction suppression in a solar flaring loop by coronal seismology of slow-mode waves

TL;DR

This study uses coronal seismology of slow-mode waves in a solar flare loop to provide evidence that thermal conduction is suppressed in hot, flaring coronal loops, affecting wave damping and energy transport.

Contribution

It demonstrates how slow-mode wave observations can quantify thermal conduction suppression and viscosity enhancement in solar flare loops, advancing coronal seismology techniques.

Findings

Thermal conductivity is suppressed by at least a factor of 3 in hot flare loops.

Classical viscosity must be enhanced by up to 15 times to explain wave damping.

Observed wave properties match a 1D linear MHD model with suppressed conduction.

Abstract

Analysis of a longitudinal wave event observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) is presented. A time sequence of 131 A images reveals that a C-class flare occurred at one footpoint of a large loop and triggered an intensity disturbance (enhancement) propagating along it. The spatial features and temporal evolution suggest that a fundamental standing slow-mode wave could be set up quickly after meeting of two initial disturbances from the opposite footpoints. The oscillations have a period of ~12 min and a decay time of ~9 min. The measured phase speed of 500$\pm$50 km/s matches the sound speed in the heated loop of ~10 MK, confirming that the observed waves are of slow mode. We derive the time-dependent temperature and electron density wave signals from six AIA extreme-ultraviolet (EUV) channels, and find that they are nearly in phase.The measured polytropic index from the temperature and density perturbations is 1.64$\pm$0.08 close to the adiabatic index of 5/3 for an ideal monatomic gas. The interpretation based on a 1D linear MHD model suggests that the thermal conductivity is suppressed by at least a factor of 3 in the hot flare loop at 9 MK and above. The viscosity coefficient is determined by coronal seismology from the observed wave when only considering the compressive viscosity dissipation. We find that to interpret the rapid wave damping, the classical compressive viscosity coefficient needs to be enhanced by a factor of 15 as the upper limit.