Magnetic Connections across the Chromosphere-Corona Transition Region

The plasma contributing to emission from the Sun between the cool chromosphere (≤104 K) and hot corona (≥106 K) has been subjected to many different interpretations. Here we look at the magnetic structure of this transition region (TR) plasma, based upon the implications of CLASP2 data of an active region recently published by Ishikawa et al., and earlier Interface Region Imaging Spectrograph (IRIS) and Solar Dynamics Observatory (SDO) data of quiet regions. Ishikawa et al. found that large areas of sunspot plages are magnetically unipolar as measured in the cores of Mg ii resonance lines, formed in the lower TR under low plasma-β conditions. Here we show that IRIS images in the line cores have fibrils that are well aligned with the overlying coronal loop segments seen in the 171 Å channel of SDO. When the TR emission in active regions arises from plasma magnetically and thermally connected to the corona, then the line cores can provide the first credible magnetic boundary conditions for force-free calculations extended to the corona. We also re-examine IRIS images of dynamic TR cool loops previously reported as a major contributor to TR emission from the quiet Sun. Dynamic cool loops contribute only a small fraction of the total TR emission from the quiet Sun.