Existence of The Closed Magnetic Field Lines Crossing The Coronal Hole Boundaries

TL;DR

This study investigates the magnetic field structures of coronal holes, revealing that many contain closed magnetic field lines crossing their boundaries, challenging the traditional association of coronal holes solely with open magnetic flux.

Contribution

The paper applies both potential-field and magnetohydrodynamic models to demonstrate the presence of boundary-crossing closed magnetic field lines in coronal holes, providing new insights into their magnetic topology.

Findings

Over 50% of potential-field and 17% of MHD coronal holes lack open magnetic field lines.

Boundary-crossing field lines are more common at lower latitudes during active periods.

Coronal holes without open field lines tend to be smaller and less unipolar.

Abstract

Coronal holes (CHs) are regions with unbalanced magnetic flux, and have been associated with open magnetic field (OMF) structures. However, it has been reported that some CHs do not intersect with OMF regions. To investigate the inconsistency, we apply a potential-field (PF) model to construct the magnetic fields of the coronal holes. As a comparison, we also use a thermodynamic magnetohydrodynamic (MHD) model to synthesize coronal images, and identify CHs from the synthetic images. The results from both the potential-field CHs and synthetic MHD CHs reveal that there is a significant percentage of closed field lines extending beyond the CH boundaries and more than 50% (17%) of PF (MHD) CHs do not contain OMF lines. The boundary-crossing field lines are more likely to be found in the lower latitudes during active times. While they tend to be located slightly closer than the non-boundary-crossing ones to the CH boundaries, nearly 40% (20%) of them in PF (MHD) CHs are not located in the boundary regions. The CHs without open field lines are often smaller and less unipolar than those with open field lines. The MHD model indicates higher temperature variations along the boundary-crossing field lines than the non-boundary crossing ones. The main difference between the results of the two models is that the dominant field lines in the PF and MHD CHs are closed and open field lines, respectively.