Direct Measurement of AIA 171 Coronal Loop Transparency

Observations suggest that coronal loops should not be completely transparent. The transparency of coronal loops is rarely investigated in spite of its key role in coronal diagnostics. Here, we present an original investigation of Atmospheric Imaging Assembly 171 coronal loop transparency directly from the radiation of visually intersecting coronal loops, which strongly indicates that the coronal loops may have significant opaqueness on the 171 Å radiation, and therefore should not be optical thin structures at least for some coronal lines. We suggest that this result may not only be helpful for explaining some basic observational features of coronal loops, but also in bringing new clues to the radiation-based diagnostics.