Andrew Leisner

The open solar magnetic field refers to field lines that are magnetically connected to the surface of the Sun at one end and continue through the corona into the heliosphere. The open magnetic flux problem is the discrepancy between the total unsigned open magnetic flux determined using in situ measurements and that derived in the corona via remote-sensing observations of the photospheric magnetic field. Recent work has shown that the missing open flux derived by the Wang–Sheeley–Arge (WSA) model is located at coronal hole boundaries, which consist of mixed regions of open and closed field lines. The method for identifying these boundaries is based on field-line tracing, which locates the footpoints of open field lines by tracing them down from the model outer boundary to the photosphere. The approach takes advantage of the intrinsic uncertainty when tracing the field at finite resolution, as it creates a randomly selected boundary surrounding the coronal hole perimeters. In this paper, we first describe an extension of previous work to include data from the Helioseismic and Magnetic Imager, which extends the period of alignment between modeled and in situ derived open flux from 28 to 35 yr. Second, by adjusting the resolution of the model we can adjust the width of the boundary region. We find that at higher resolutions the total open flux predicted decreases, while if the resolution is reduced, the open flux increases. Furthermore, between 1° and 2° the total open flux prediction matches the in situ measurements most closely.

The interplanetary magnetic field (IMF) measured near Earth can be up to 2 times greater than that derived from models using remote solar observations. We investigate this discrepancy by modeling the IMF using a potential field source surface (PFSS) model using synoptic maps of the photospheric magnetic field from 2010 May to 2024 April. Five types of radial field synoptic maps are used in this work: the B _r synoptic maps from vector magnetic field data, the M _r synoptic maps from the line-of-sight field data, the rescaled M _r synoptic maps rescaled from the M _r maps by a center-to-limb distance dependent rescaling factor of B _r / M _r , and composite and rescaled composite synoptic maps comprised of a combination of strong-field pixels from the B _r maps and the rest from either the original M _r or rescaled M _r maps. The modeled IMFs from all five types of synoptic maps agree with each other well in the solar maximum phase, when they are about 2 times smaller than in situ measurements. The IMF calculated from the B _r and both composite and rescaled composite synoptic maps match well with in situ observations during solar minimum from 2017 to 2022. The IMF values modeled from both the M _r and rescaled M _r synoptic maps are still significantly smaller in this time interval. This suggests that (1) the B _r maps represent the radial field better than the M _r ; and (2) the PFSS model is appropriate to model the heliospheric magnetic field in solar minimum, but has limitations when used near solar maximum.

Coronal holes (CHs) are magnetically open regions that allow hot coronal plasma to escape from the Sun and form the high-speed solar wind. This wind can interact with Earth’s magnetic field. For this reason, developing an accurate understanding of CH regions is vital for understanding space weather and its effects on Earth. The process of identifying CH regions typically relies on extreme ultraviolet (EUV) imagery, leveraging the fact that CHs appear dark at these wavelengths. Accurate identification of CHs in EUV, however, can be difficult due to a variety of factors, including stray light from nearby regions, limb brightening, and the presence of filaments (which also appear dark, but are not sources of solar wind). In order to overcome these issues, this work incorporates photospheric magnetic-field data into a classical EUV-based segmentation algorithm based on the Active Contours Without Edges (ACWE) segmentation method. In this work magnetic-field data are incorporated directly into the segmentation process, serving both as a method for removing non-CH regions in advance, and as a method to constrain evolution of the segmented CH boundary. This reduces the presence of filaments while allowing the segmentation to include CH regions that may be difficult to identify due to inconsistent intensities.

The solar magnetic fields emerging from the photosphere into the chromosphere and corona are comprised of a combination of closed (field lines with both ends rooted at the Sun) and open (field lines with only one end at the Sun) fields. Since the early 2000s, the magnitude of total unsigned open magnetic flux estimated by coronal models has been in significant disagreement with in situ spacecraft observations, especially during solar maximum. Estimates of total open unsigned magnetic flux using coronal hole observations (e.g., using extreme ultraviolet or helium (He) I) are in general, in average agreement with the coronal model results and thus show similar disagreements with in situ observations. This paper provides a brief overview of the problem, summarizes the proposed explanations for the discrepancies, and presents results that strongly support the explanation that the discrepancy is due to dynamics at the open-closed boundary. These results are derived from the determination of the total unsigned open magnetic flux, utilizing the Wang–Sheeley–Arge model at a particular spatial resolution and different field-line tracing methods. One of these methods produces excellent agreement with in situ observations. Our results imply that strong magnetic fields in close proximity to active regions and residing near the boundaries of mid-latitude coronal holes are the primary source of the missing open flux. Furthermore, the results outlined here resolve many of the seemingly contradictory facts that have made the open-flux problem so difficult.

Automated detection schemes are nowadays the standard approach for locating coronal holes in extreme-UV images from the Solar Dynamics Observatory (SDO). However, factors such as the noisy nature of solar imagery, instrumental effects, and others make it challenging to identify coronal holes using these automated schemes. While discrepancies between detection schemes have been noted in the literature, a comprehensive assessment of these discrepancies is still lacking. The contribution of the Coronal Hole Boundary Working Team in the COSPAR ISWAT initiative to close this gap is threefold. First, we present the first community data set for comparing automated coronal hole detection schemes. This data set consists of 29 SDO images, all of which were selected by experienced observers to challenge automated schemes. Second, we use this community data set as input to 14 widely applied automated schemes to study coronal holes and collect their detection results. Third, we study three SDO images from the data set that exemplify the most important lessons learned from this effort. Our findings show that the choice of the automated detection scheme can have a significant effect on the physical properties of coronal holes, and we discuss the implications of these findings for open questions in solar and heliospheric physics. We envision that this community data set will serve the scientific community as a benchmark data set for future developments in the field.

Coronal Holes (CHs) are regions of open magnetic-field lines, resulting in high-speed solar wind. Accurate detection of CHs is vital for space-weather prediction. This paper presents an intramethod ensemble for coronal-hole detection based on the Active Contours Without Edges (ACWE) segmentation algorithm. The purpose of this ensemble is to develop a confidence map that defines, for all ondisk regions of a solar extreme ultraviolet (EUV) image, the likelihood that each region belongs to a CH based on that region’s proximity to, and homogeneity with, the core of identified CH regions. By relying on region homogeneity, and not intensity (which can vary due to various factors, including line-of-sight changes and stray light from nearby bright regions), to define the final confidence of any given region, this ensemble is able to provide robust, consistent delineations of the CH regions. Using the metrics of global consistency error (GCE), local consistency error (LCE), intersection over union (IOU), and the structural similarity index measure (SSIM), the method is shown to be robust to different spatial resolutions maintaining a median IOU $>0.75$ > 0.75 and minimum SSIM $>0.93$ > 0.93 even when the segmentation process was performed on an EUV image decimated from $4096\times 4096$ 4096 × 4096 pixels down to $512\times 512$ 512 × 512 pixels. Furthermore, using the same metrics, the method is shown to be robust across short timescales, producing segmentation with a mean IOU of 0.826 from EUV images taken at a 1-h cadence, and showing a smooth decay in similarity across all metrics as a function of time, indicating self-consistent segmentations even when corrections for exposure time have not been applied to the data. Finally, the accuracy of the segmentations and confidence maps are validated by considering the skewness (i.e., unipolarity) of the underlying magnetic field.

The solar magnetic fields emerging from the photosphere into the chromosphere and corona are comprised of a combination of "closed" and "open" fields. The closed magnetic field lines are defined as those having both ends rooted in the solar surface, while the open field lines are those having one end extending out into interplanetary space and the other rooted at the Sun's surface. Since the early 2000's, the amount of total unsigned open magnetic flux estimated by coronal models have been in significant disagreement with in situ spacecraft observations, especially during solar maximum. Estimates of total open unsigned magnetic flux using coronal hole observations (e.g., using extreme ultraviolet (EUV) or Helium (He) I) are in general agreement with the coronal model results and thus show similar disagreements with in situ observations. While several possible sources producing these discrepancies have been postulated over the years, there is still no clear resolution to the problem. This paper provides a brief overview of the problem and summarizes some proposed explanations for the discrepancies. In addition, two different ways of estimating the total unsigned open magnetic flux are presented, utilizing the Wang-Sheeley-Arge (WSA) model, and one of the methods produce surprisingly good agreement with in situ observations. The findings presented here suggest that active regions residing near the boundaries of mid-latitude coronal holes are the probable source of the missing open flux. This explanation also brings in line many of the seemly contradictory facts that have made resolving this problem so difficult.