Unipolarity of the solar magnetic field in equatorial coronal holes

TL;DR

This study analyzes the magnetic polarity imbalance in equatorial coronal holes using HMI data, revealing significant unipolar dominance, stable magnetic distributions over time, and a moderate link to high-speed solar wind streams, highlighting the role of magnetic flux imbalance in plasma acceleration.

Contribution

It provides the first detailed statistical analysis of magnetic polarity skewness and flux imbalance in equatorial coronal holes and their relation to solar wind speeds.

Findings

88% of coronal holes show magnetic polarity dominance

Flux imbalance ranges from 20% to 45% in coronal holes

Moderate correlation (r=0.60) between flux imbalance and solar wind speed

Abstract

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 ($\Phi_{\mathrm{imb}}$) 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 ($\Phi_{\mathrm{imb}}$), 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.