The Transition Region of Solar Flare Loops

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 and extreme ultraviolet (EUV) observations. A major result of these studies is that the amount of plasma at low temperatures, <10(5) 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 a problem in the theory. We address this issue by analyzing high-spatial and temporal resolution EUV observations from an X1.6-class flare, taken with the Interface Region Imaging Spectrograph and the Solar Dynamic Observatory/Atmospheric Imaging Assembly (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 1334.525 angstrom and Si iv 1402.770 angstrom transition region spectral lines, the Fe xxi 1354.066 angstrom flare line, and the AIA 171 angstrom 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.