Examination of the EUV Intensity in the Open Magnetic Field Regions Associated with Coronal Holes

Coronal holes can be identified as the regions with magnetic field lines extending far away from the Sun, or the darkest regions in EUV/X-ray images with predominantly unipolar magnetic fields. A comparison between the locations of our determined regions with open magnetic field lines (OMF) and regions with low EUV intensity (LIR) reveals that only 12% of the OMF regions coincide with the LIRs. The aim of this study is to investigate the conditions leading to the different brightnesses of OMF regions, and to provide a means to predict whether an OMF region would be bright or dark. Examining the statistical distribution profiles of the magnetic field expansion factor (f_s) and Atmospheric Imaging Assembly 193 Å intensity (I₁₉₃) reveals that both profiles are approximately log-normal. The analysis of the spatial and temporal distributions of f_s and I₁₉₃ indicates that the bright OMF regions often are inside or next to regions with closed field lines, including quiet-Sun regions and regions with strong magnetic fields. Examining the relationship between I₁₉₃ and f_s reveals a weak positive correlation between log I₁₉₃ and log f_s , with a correlation coefficient ≈ 0.39. As a first-order approximation, the positive relationship is determined to be log I₁₉₃ = 0.62 log f_s + 1.51 based on the principle of the whitening/dewhitening transformation. This linear relationship is demonstrated to increase the consistency between the OMF regions and LIRs from 12% to 23%.