Unipolarity of the solar magnetic field in equatorial coronal holes

TL;DR

This work investigates the unipolar nature of coronal holes by quantifying the photospheric magnetic field under 70 holes with the LOS component $B_{ ext{LOS}}$ from HMI, using skewness $S$ and magnetic flux imbalance $Φ_{ ext{imb}}$ as primary metrics. It combines SDO/AIA-based boundary detection in the 193 Å channel with HMI magnetograms and correlates the derived unipolarity with 1 AU high-speed solar wind streams, after filtering out ICMEs and CIRs. The study finds typical CH skewness in the range $0.20$–$0.40$ and flux imbalance between $20\%$–$45\%$, while quiet Sun regions show far smaller asymmetry; a strong $|S|$–$Φ_{ ext{imb}}$ correlation ($r \approx 0.99$) and a moderate $Φ_{ ext{imb}}$–$v_{ ext{HSS}}$ correlation ($r \approx 0.60$) indicate that unipolar open flux facilitates faster solar wind, though other factors also influence wind speed. These results provide a practical diagnostic for CH identification and enhance the understanding of how coronal-hole magnetic topology relates to space-weather-relevant solar wind acceleration.

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 ($Φ_{\mathrm{imb}}$) 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.

Figures (8)

• Figure 1: The left panel 'a' displays a coronal hole observed by SDO/AIA 193 Å on 2015-07-07 at 11:44:41, centered on the solar disk. The corresponding photospheric magnetogram is shown in the right panel 'b', as observed by SDO/HMI, depicting the line-of-sight magnetic field ($B_{\mathrm{LOS}}$). The black dotted box indicates the region of interest.
• Figure 2: In-situ solar wind and magnetic field parameters associated with the coronal hole (Figure \ref{['fig:Figure_1_AIA_HMI_Aligned_ROI_map']}). Panels (top to bottom) display: magnetic field components ($B_x$, $B_y$, $B_z$) with total magnitude ($|B|$), solar wind speed ($V$) with proton temperature ($T_p$), proton density ($n_p$) with pressure ($P$), and the SYM-H index. The vertical red dashed lines indicate the time interval of the constant phase of the high-speed solar wind stream used for averaging (OMNIWeb, NASA GSFC).
• Figure 3: Left: Histogram of skewness distribution for the quiet sun regions (blue) and coronal holes (red). This graph illustrates how skewness varies between these two solar features. Right: Histogram of magnetic flux imbalance for the quiet sun regions(blue) and coronal holes (red). This graph shows the differences in magnetic flux imbalance between these regions.
• Figure 4: An asymmetric distribution of the photospheric $B_{\mathrm{LOS}}$ within the coronal hole is observed across the solar disk, spanning from 2015-07-03 (near the east limb) to 2015-07-10 (near the west limb). Each red dot, with coordinates in arcseconds, represents the centroid position of the coronal hole as it moves across the disk.
• Figure 5: The left panel 'a' shows the correlation plot between absolute magnetic skewness and magnetic flux imbalance, while the right panel 'b' presents the correlation plot between magnetic flux imbalance and the HSS speed ($v_{\mathrm{HSS}}$)