The Sun's open-closed flux boundary and the origin of the slow solar wind

Nugget
Number:	497
1st Author:	Chloe WILKINS
2nd Author:	David PONTIN
Published:	May 26, 2025
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Introduction

There is a definite boundary in the solar magnetic field between "open" (connecting into the solar wind) and "closed" (coronal loops) magnetic flux on the Sun. This open-closed boundary or OCB is a dynamic interface where "interchange" magnetic reconnection can occur. Observations indicate that the slow solar wind (SSW) and closed corona share similar ratios of elements with low-to-high first ionisation potentials (e.g., Ref. [1]), suggesting that the SSW plasma originates within closed magnetic flux, but is subsequently transported into the heliosphere along open field lines. The continual exchange of open and closed flux by interchange magnetic reconnection at the OCB is therefore thought to be linked to the origin of the slow wind.

In our study (Ref. [2]), we analyse the OCB over the solar cycle and quantify the open magnetic flux that lies within one supergranular diameter of the OCB. We compare this to interplanetary scintillation (IPS) observations of the solar wind and explore the implications of our results for understanding its origins.

Key Results

The Sun's magnetic field evolves over an 11-year solar cycle, from a field largely dominated by a dipole component at solar minimum, to a more complex field at solar maximum (see the top panel of Figure 1). It is well known that the total quasi-steady open flux measured in the heliosphere is considerably larger during solar maximum than during solar minimum (Ref. [3]). To characterise the Sun's OCB, it is therefore important to explore the magnetic field structure across different phases of the solar cycle. We focus on the period December 2010 to December 2019 within Solar Cycle 24, and model the Sun's global coronal magnetic field using mathematical extrapolations with increasing sophistication:

• a potential field source surface (PFSS) model;

• a static equilibrium magnetofrictional (SEMF) model; and

• a time-dependent magnetofrictional (TDMF) model with surface flux transport.

Figure 1: Top: a subset of field lines for the TDMF model about (left) solar maximum and (right) solar minimum (April 2014 and December 2019, respectively). The signed logarithm (slog(Q)) of the "squashing factor" Q, which maps the gradients of the field connectivity, is plotted on the photosphere and the plots are centred at a longitude of φ=180° with closed (open) field coloured green (purple). Middle: slog(Q) maps of latitude θ against longitude φ overlaid with a black contour representing the OCB. Bottom: open field within 25 Mm of the OCB on the photosphere, coloured according to signed flux-per-pixel in Maxwells.

For each model, we compute the open magnetic flux that lies within 25 Mm of the OCB, as shown in the bottom panel of Figure 1, and determine the fraction of the "source surface" of the solar wind that this flux occupies. As shown in the bottom panel of Figure 2, this fraction reaches ~90% near solar maximum and declines to ~40% at solar minimum. Importantly, we find that this open-flux fraction is consistent across all magnetic field models, regardless of whether they include surface flux transport or account for non-potentiality.

Figure 2: Open flux calculations for PFSS (cyan), SEMF (orange), TDMF (purple) models and PFSS extrapolations of HMI data (black). Top panel: total unsigned open flux over the entire solar surface. Second: total unsigned open flux within 25 Mm of the OCB on the photosphere (the near-OCB open flux). Third: the percentage of near-OCB open flux to the total open flux on the photosphere. Bottom: the percentage of area occupied by the near-OCB open flux at the source surface. The red (blue) shaded regions indicate solar maximum (minimum).

We find a strong agreement across the solar cycle between this open-flux fraction and the fraction of SSW (<530 km/s) inferred from IPS observations (Ref. [4]). During solar maximum - where the SSW fraction peaks at ~80% - we find that nearly all open flux lies within 25 Mm of the OCB.

Conclusions

Our analysis of the open magnetic flux near the OCB reinforces the proposed link between interchange reconnection and the origin of the SSW. The strong correlation between the observed fraction of slow wind and the fraction of open flux within one supergranular diameter of the OCB supports the idea that this boundary region plays a key role in transporting interchange-released plasma into the heliosphere.

Importantly, the open-flux fraction remains consistent across a range of coronal magnetic field models. This suggests that even simple, computationally inexpensive PFSS extrapolations can be used to predict SSW occurrence and variability over the solar cycle.

Acknowledgments

This work (and Ref. [2]) is the result of a collaborative effort between the authors and Anthony R. Yeates, Spiro K. Antiochos, Hannah Schunker, and Bishnu Lamichhane.

References

[1] "A Model for the Sources of the Slow Solar Wind"

[2] "The Sun's open-closed flux boundary and the origin of the slow solar wind"

[3] "Proposed Resolution to the Solar Open Magnetic Flux Problem"

[4] "Global Distribution of the Solar Wind Speed Reconstructed from Improved Tomographic Analysis of Interplanetary Scintillation Observations between 1985 and 2019"