SOLARNET Conference: The Many Scales of the Magnetic Sun

Solar coronal holes (CHs) are the regions with low coronal emissions and an unbalanced magnetic flux, which led to the common expectation that their magnetic structures are mostly ``open'', or, in other words, extending to infinity. The objective of this study is to examine the CH magnetic structures to verify this expectation. The coronal holes are identified from the synoptic maps constructed using the AIA 193 images and HMI line-of-sight magnetograms from 2010 June to 2020 February. The magnetic structures of the CHs are constructed using the Potential Field Source Surface (PFSS) model. As a comparison, the data from a thermodynamic magnetohydrodynamic (MHD) model are also used. The results from both models reveal that there is a significant percentage of closed field lines extending beyond the CH boundaries, leading to an unbalanced magnetic flux within the boundaries. The boundary-crossing field lines tend to be located slightly closer to the CH boundaries, and are more likely to be found in the lower latitudes during active times. The results also show that more than 50% (17%) of PF (MHD) CHs do not contain open magnetic field lines. The CHs without open field lines are often smaller and less unipolar than those with open field lines.