The Transition Region of Solar Flare Loops

TL;DR

This study uses high-resolution EUV observations of a solar flare to show that the standard thermal conduction model in the transition region of flare loops is invalid, suggesting turbulence may suppress conduction.

Contribution

It provides observational evidence challenging the standard conduction model in flare loops, proposing turbulence as a possible explanation.

Findings

EM ratios incompatible with standard conduction model

Large EM increases due to flare heating are observed

Standard Spitzer-Harm thermal conductivity is likely invalid

Abstract

The transition region between the Sun's corona and chromosphere is important to the mass and energy transfer from the lower atmosphere to the corona; consequently, this region has been studied intensely with ultraviolet (UV) and extreme ultraviolet observations. A major result of these studies is that the amount of plasma at temperatures smaller than 100 000 K, is far too large to be compatible with the standard theory of thermal conductivity. However, it is not clear whether the disagreement lies with a problem in the observations or in the theory. We address this issue by analysing high-spatial and temporal resolution EUV observations from an X1.6-class flare taken with the Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). These data allow us to isolate the emission of flare loops from that of surrounding structures. We compare the Emission Measures (EMs) derived from the C II 1334.525Ang., Si IV 1402.770Ang transition region spectral lines, the Fe XXI 1354.066Ang flare line and the AIA 171Ang coronal images. We find that the EM ratios are incompatible with a standard conduction-dominated transition region model. Furthermore, the large increases in the EM magnitudes due to flare heating make it highly unlikely that the disagreement between data and theory is due to observational uncertainties in the source of the emission. We conclude that the standard Spitzer Harm thermal conductivity must be invalid for, at least, flare loops. We discuss the possibility that turbulent suppression of thermal conduction can account for our results.