  Previous studies have found that coronal loops have a nearly uniform
thickness, which seems to disagree with the characteristic expansion of active
region magnetic fields. This is one of the most intriguing enigmas in solar
physics. We here report on the first comprehensive one-to-one comparison of
observed loops with corresponding magnetic flux tubes obtained from cotemporal
magnetic field extrapolation models. We use EUV images from TRACE, magnetograms
from the MDI instrument on SOHO, and linear force-free field extrapolations.
For each loop, we find the particular value of the force-free parameter (alpha)
that best matches the observed loop axis and then construct flux tubes using
different assumed cross sections at one footpoint (circle and ellipses with
different orientations). We find that the flux tubes expand with height by
typically twice as much as the corresponding loops. We also find that many flux
tubes are much wider at one footpoint than the other, whereas the corresponding
loops are far more symmetric. It is clear that the actual coronal magnetic
field is more complex than the models we have considered. We suggest that the
observed symmetry of loops is related to the tangling of elemental magnetic
flux strands produced by photospheric convection.
