  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.
