Abstract Non-Linear Force-Free Fields (NLFFF) play a key role in our understanding of the nature and evolution of coronal magnetic fields. Two of the most common methods for their construction are the “extrapolation” and “evolution” approaches. The aim of the present paper is to compare results from these two approaches when they have the same vector magnetic field on the bottom boundary. To begin with a NLFFF evolution simulation of AR10977 is carried out to produce a time series of the vector field at the lower boundary covering the full life-span of the active region. Next at eight unique times in this time series, NLFFF extrapolations are constructed using the simulated vector boundary data. The resulting 3D coronal magnetic fields are then compared. During the early stages in the lifetime of AR10977, when the coronal magnetic field is composed of simply connected field lines, both NLFFF approaches produce a high level of agreement as long as the full vector field is injected into the extrapolation. When injection is limited to only strong field locations, a poorer agreement is found. In contrast, once a flux rope has formed during the later stages in the lifetime of the active region poor agreement is found between the two approaches, regardless of how the boundary information is injected in the extrapolations. This indicates that once a flux rope has formed through flux cancellation and risen into the corona the information held within the boundary vector field is insufficient to capture the complexity of the 3D coronal magnetic field. This result is also supported by the poor agreement that arises when comparing the relative magnetic helicity between the two modelling approaches. While the present study considers one extrapolation approach, it is important to repeat the study using alternative extrapolation methods that exist in the published literature.

Abstract The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1 + 3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5° (6 R ⊙ ) to 8° (32 R ⊙ ); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3° (12 R ⊙ ) to 45° (180 R ⊙ ) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single “virtual instrument” with a 90 ∘ wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

Extreme-ultraviolet (EUV) images from the Atmospheric Imaging Assembly on the Solar Dynamics Observatory and the EUV Imager on the Solar Terrestrial Relations Observatory show that coronal hole boundaries often change from one day to the next on spatial scales up to several supergranules. Such changes may occur even in the absence of nearby sunspots or transient activity. We attribute the fluctuations to the action of supergranular convection, which continually rearranges the photospheric flux distribution both near and far from the hole boundaries. The boundary displacements may exceed a supergranular diameter because, in addition to simple advection, the open magnetic flux may undergo interchange reconnection with the long closed loops rooted just outside the boundary. This injects streamer material into the heliospheric plasma sheet but does not lead to a mixing of open and closed flux, whose interface remains clearly defined in EUV images and qualitatively consistent with current-free extrapolations of the (instantaneous) photospheric field. However, the boundary fluctuations are likely to be a major cause of the well-known variability of the slow solar wind, with the footpoint locations of the wind intercepted by a given spacecraft continually changing relative to the hole boundary on timescales of a day or less. This variability reflects the steep increase in the rate of flux-tube divergence toward the boundary, which leads to rapid changes in the measured wind speeds and densities. We also describe an unusual case in which a long-lived coronal hole forms suddenly without any nearby flux emergence, apparently as a result of transient-driven interchange reconnection with the north polar hole.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11207-026-02610-8.

We present the Brightness–Location (BriLo) method, a novel single-spacecraft technique which exploits the Thomson scattering theory for localizing extended coronal features such as streamers using white-light (WL) imaging. Beyond determining the longitude and latitude of coronal features, the method also provides estimates of their geometrical properties, such as angular width (column depth). Validation is performed through geometrical triangulation with multi-viewpoint coronagraphs (the Solar TErrestrial RElations Observatory A COR2 and the Solar and Heliospheric Observatory C2–C3). The method is applied to ten coronal streamers observed by the Wide-Field Imager for Solar Probe (WISPR) on board the Parker Solar Probe (PSP) between encounter 1 – 17. We applied BriLo to two different data products, L3 and LX, which differ in K-corona treatment and absolute brightness levels. The L3 and LX results show good agreement in deriving streamer directionality, with differences of 2 – 30° in longitude and 1 – 6° in latitude. Both datasets provide longitude and latitude estimates that are broadly consistent with triangulation results. We further classified streamers and compared their locations with potential-field source surface (PFSS) extrapolations of the heliospheric current sheet (HCS). Helmet streamers are generally found close to the HCS, whereas pseudostreamers in proximity to active regions. In conclusion, the application of BriLo to LX data yields realistic streamer widths of several to ten degrees, while L3 data produce unrealistically narrow values below one degree. This discrepancy arises from the line of sight (LOS) integration of the observed signal and the dependence of F-corona removal on background estimation and coronal conditions. Overall, BriLo proves to be a robust tool not only for streamer localization but also for assessing and validating WL imaging techniques.

Abstract The complex nature of sunspots presents a constant challenge in understanding their dynamics and how they interact with their surrounding environment. A new method to analyse sunspot rotations has been presented, by which the rotation of sunspots can be tracked using multiple ellipses fitted throughout the sunspot umbrae. The method is applied to sunspots in active regions of differing degrees of flaring, to determine a correlation between the rotation of the sunspot and the appearance of a flare. The study reveals that sunspots in active regions associated with flares present complex patterns of rotation within the umbral plasma, where multiple regions of rotation can be observed. Further implications of this study could help determine the link between sunspots and flares, which will contribute towards the betterment of flare forecasting.