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The plasma contributing to emission from the Sun between the coolchromosphere ($\le 10^4$K) and hot corona ($\ge 10^6$K) has been subjected tomany different interpretations. Here we look at the magnetic structure of thistransition region (TR) plasma, based upon the implications of CLASP2 data of anactive region recently published by Ishikawa et al., and earlier IRIS and SDOdata of quiet regions. Ishikawa et al. found that large areas of sunspot plagesare magnetically unipolar as measured in the cores of \ion{Mg}{2} resonancelines, formed in the lower transition region under low plasma-$\beta$conditions. Here we show that IRIS images in the line cores have fibrils whichwell aligned with the overlying coronal loop segments seen in the 171 \AA{}channel of SDO. When the TR emission in active regions arise from plasmamagnetically and thermally connected to the corona, then the line cores canprovide the first credible magnetic boundary conditions for force-freecalculations extended to the corona. We also re-examine IRIS images of dynamicTR cool loops previously reported as a major contributor to transition regionemission from the quiet Sun. Dynamic cool loops contribute only a smallfraction of the total TR emission from the quiet Sun.

Magnetic connections across the chromosphere-corona transition region