  Active region EUV loops are believed to trace a subset of magnetic field
lines through the corona. Malanushenko et al. (2009) proposed a method, using
loop images and line-of-sight photospheric magnetograms, to infer the
three-dimensional shape and field strength along each loop. McCarthy et al.
(2019) used this novel method to compute the total magnetic flux
interconnecting a pair of active regions observed by SDO/AIA. They adopted the
common assumption that each loop had a circular cross section. The accuracy of
inferred shape and circularity of cross sections can both be tested using
observations of the same loops from additional vantage points as provided by
STEREO/EUVI. Here, we use multiple viewing angles to confirm the
three-dimensional structure of loops. Of 151 viable cases, 105 (69.5%) matched
some form of visible coronal structure when viewed approximately in quadrature.
A loop with a circular cross-section should appear of a similar width in
different perspectives. In contradiction to this, we find a puzzling lack of
correlation between loop diameters seen from different perspectives, even an
anti-correlation in some cases. Features identified as monolithic loops in AIA
may, in fact, be more complex density enhancements. The 30.5% of
reconstructions from AIA which did not match any feature in EUVI might be such
enhancements. Others may be genuine loop structures, but with elliptical cross
sections. We observe an anti-correlation between diameter and brightness,
lending support to the latter hypothesis. Of 13 suitable for width analysis,
four loops are consistent with non-circular cross sections, where we find
anti-correlation in both comparisons.
