Difficulties in Observing Coronal Structure

There has developed in recent years a substantial body of evidence to indicate that the temperature and density structure of the corona are far more complicated than had previously been thought. We review some of the evidence and discuss some specific examples: observations of a limb flare, showing that the cool H-alpha material is cospatial with the hot X-ray emitting material: simultaneous NIXT and Yohkoh SXT observations of an active region, showing that loops seen in one instrument are not seen in the other, and that the effect works in both directions; comparisons of extrapolated magnetic field measurements to the observed coronal structure, indicating that neither potential nor constant-e force-free fits are adequate. We conclude with a description of two new instruments, the TRACE and the TXI, which will help to resolve some of these difficulties.