| dc.description.abstract | Coronal holes are persistent and recurrent features in the solar corona. They are observationally defined as dark patches with predominantly unipolar magnetic field, or theoretically as regions with magnetic field lines extending far into the interplanetary space (＂open＂ magnetic field regions). The two definitions, however, do not always coincide with each other. In this study, the first definition will be referred to as coronal hole (CH) and the second definition as open magnetic field (OMF) region. We aim to quantify the difference between the two physical phenomena, investigate the sources of inconsistencies between the two, and study the long-term evolution of the OMF regions.
The CHs are extracted from the synoptic maps constructed using the magnetic field and extreme ultraviolet (EUV) images from the Solar Dynamics Observatory (SDO). The time period of the synoptic maps spans from the Carrington Rotation (CR) number 2099 to CR2227. The extracted CHs are compared with the OMF regions constructed from potential field model, linear force-free model, and thermal magnetohydrodynamic (MHD) model. The OMF regions from the MHD model are also compared with the CHs extracted from the thermal MHD data for theoretical analysis.
By comparing the CHs and the OMF regions with the high-speed solar wind streams (HSSs), we found that the HSS source regions are more consistent with the OMF regions than with CHs. The inconsistency between the CHs and the OMF regions can come from two sources: OMF regions with high EUV intensities and CHs with closed magnetic field structures. Our analysis shows that (1) the EUV intensity is approximately positively correlated with the expansion factor of magnetic flux in logarithmic scale, which means that an OMF region with a sufficiently large expansion factor is likely to be bright in EUV images, and therefore not be qualified as a CH; (2) the closed magnetic field lines can cross the CH boundaries to connect with non-CH regions or with different CHs, thereby contributing a non-negligible amount of unipolarity to the CHs; (3) the long-term evolution of the OMF regions forms a pole-to-pole trans-equatorial migration pattern, and the speed of the migration is comparable to the measured meridional flow speed. The data used in this study, including the EUV and magnetic field synoptic maps, the synthetic EUV maps, the identified coronal holes, and the identified high-speed solar wind streams, are distributed on https://doi.org/10.34740/kaggle/ds/2783160. | en_US |