Unipolarity of the solar magnetic field in equatorial coronal holes

A study of the unbalanced magnetic polarity distribution of 70 coronal holes was performed. Data from the Helioseismic and Magnetic Imager (HMI) were used to examine the photospheric line-of-sight magnetic field ($B_{\mathrm{LOS}}$) beneath these coronal holes. The skewness ($S$) values of the $B_{\mathrm{LOS}}$ distributions revealed significant asymmetry, characterized by the dominance of one magnetic polarity, with $\sim88%$ of the coronal holes exhibiting a skewness value ranging from $\pm(0.20~\text{to}~0.40)$. The corresponding magnetic flux imbalance ($Φ_{\mathrm{imb}}$) ranges from $20%$ to $45%$. In contrast, quiet-Sun regions show symmetric magnetic field distributions with skewness values less than$~0.11$ and flux imbalance less than $11.0%$. A study of a coronal hole as it traverses across the disk shows that the magnetic field distribution does not evolve significantly over this time, remaining stable across half a solar rotation. A moderate correlation ($r = 0.60$) between the magnetic flux imbalance and the speed of associated high speed solar wind streams ($v_{\mathrm{HSS}}$) suggests that flux imbalance may contribute to the generation of these faster solar wind streams. These results imply that regions with higher flux imbalance ($Φ_{\mathrm{imb}}$), indicative of more open magnetic field structures, present more favorable conditions for plasma acceleration as compared to closed bi-polar field, but the moderate correlation indicates that other factors may also play important roles.

💡 Research Summary

This paper presents a comprehensive statistical analysis of the magnetic polarity distribution within equatorial coronal holes, providing strong empirical evidence for their theorized unipolar, open-field nature. The study investigates 70 isolated coronal holes observed near the solar disk center between 2010 and 2020 (Solar Cycle 24).

The methodology combines remote-sensing and in-situ data. Coronal hole boundaries are defined using intensity thresholds applied to 193 Å images from the Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA). The photospheric magnetic field beneath these regions is analyzed using line-of-sight magnetogram data (B_LOS) from SDO’s Helioseismic and Magnetic Imager (HMI). The key metric for quantifying magnetic unipolarity is the skewness (S) of the B_LOS distribution within each hole. A skewness of zero indicates a balanced, symmetric distribution, while positive or negative values signify a dominance of positive or negative magnetic polarity, respectively. In-situ solar wind data from the OMNI database, filtered to exclude coronal mass ejections and stream interaction regions, are used to link coronal hole properties to high-speed solar wind streams (HSS) observed at 1 AU.

The core findings are threefold. First, the B_LOS distributions in coronal holes are highly asymmetric. Approximately 88% of the studied holes exhibit skewness values in the range of ±0.20 to ±0.40, corresponding to a magnetic flux imbalance (Φ_imb) between 20% and 45%. This starkly contrasts with quiet-Sun regions, which show nearly symmetric distributions with skewness < 0.11 and flux imbalance < 11%. This result quantitatively confirms that coronal holes are fundamentally unipolar magnetic structures. Second, a case study tracking a single coronal hole over half a solar rotation demonstrates that this skewed magnetic distribution is a stable, persistent feature of the hole, not a transient phenomenon. Third, the analysis reveals a moderate positive correlation (r = 0.60) between the degree of magnetic flux imbalance in a coronal hole and the speed of its associated high-speed solar wind stream.

The study concludes that the significant flux imbalance observed in coronal holes is a signature of their open magnetic field configuration, where one magnetic polarity dominates and field lines extend freely into interplanetary space. This open structure is more conducive to plasma acceleration than closed, bipolar fields, explaining the moderate correlation with solar wind speed. The open field acts as a preferential channel for the fast solar wind. However, the correlation is not perfect, indicating that while flux imbalance is an important factor, other mechanisms—such as coronal heating processes, wave-particle interactions, or the detailed fine-scale structure within the hole—also play crucial roles in the final acceleration of the solar wind. This work provides robust observational benchmarks for models of coronal hole magnetism and its connection to solar wind generation, contributing to a more precise understanding of space weather origins.