The Cross-Field Thermal Structure of Coronal Loops from Triple-Filter TRACE Observations

The highly suppressed thermal transport across the magnetic field in the solar corona makes the determination of the cross-field thermal distribution within coronal loops a powerful diagnostic of the properties of the heating process itself. The cross-field thermal structure is currently being strongly debated. Spectroscopic observations with high temperature fidelity but low spatial resolution indicate that some observed loops are multithermal, whereas imaging observations with high spatial resolution but low temperature fidelity indicate more isothermal conditions. We report here on triple filter observations of coronal loops made by the Transition Region and Coronal Explorer (TRACE), which has the best spatial resolution currently available. We tested the isothermal hypothesis using the emission measure loci technique and found that the loops are consistent with an isothermal plasma near 1.5 MK only if a generous estimate of the photometric uncertainties is used. A more restrictive estimate based on discussions with the TRACE experimenters rules out the isothermal hypothesis. The observations are much better explained by a multithermal plasma with significant emission measure throughout the range 1-3 MK. The details of the emission measure distribution are not well defined, however. Future subarcsecond spectroscopic observations covering a wide range of temperatures are the most promising means of unlocking the thermal structure of the corona.