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.

The WISPR imager on Parker Solar Probe (PSP) provides a unique view of the young solar wind as it flies through solar wind structures at high speed. It is of interest to use WISPR image sequences to measure the velocity of both large features (such as coronal mass ejections) and the background, ambient wind. However, WISPR’s close-up, rapidly moving perspective makes the usual methods for measuring velocities from images difficult or impossible to apply, as most apparent motion through the image is due to the motion or rotation of the imager. In this work, we propose a new method of looking for features at the “stationary point”—a direction from which some plasma parcels appear to approach the spacecraft, remaining at a constant direction in the image sequence. This direction is a function of the plasma’s radial velocity, the encounter geometry, and the spacecraft velocity, allowing the former two to be inferred. We demonstrate the technique with forward-modeled images, and we apply it to WISPR observations, inferring the speed and trajectory of a particular density feature. This method promises to enable speed measurements of the young solar wind in an important acceleration region, from a close-up perspective and at latitudes well outside the PSP orbital plane. And while we present this method in a solar wind context, it is broadly applicable to any situation of a moving viewpoint traveling through an expanding cloud of features.

We present a first-principles analytic treatment of modern multivane occulters in circular (coronagraph) and linear (heliospheric imager) geometry, develop a simplified theory that is useful for designing and predicting their performance, explain certain visual artifacts, and explore the performance limits of multivane occulters. Multivane occulters are challenging to design in part because they violate the conditions for both the Fraunhofer and Fresnel approximations to diffraction theory, and new designs have therefore generally required explicit simulation, empirical measurement, “guesstimation,” or all three. Starting from the Kirchoff diffraction integral, we develop a “successive plane-wave” approximate analytic theory that is suitable for predicting the performance of multivane occulters, and use it to derive closed-form expressions for the performance of new designs. We review the fundamental 2D system of an occulter edge, discuss how it applies to real 3D systems by extrusion or revolution, present the reason for observed bright quasi-achromatic fringing around coronagraph occulters, develop the successive plane wave approximation in 2D and explore its limits, describe the relevance of the 2D theory to practical 3D instruments, and discuss implications for multivane occulter design in current and future instruments.

IntroductionCoronal Faraday rotation (FR) measurements provide a powerful means of measuring the magnetic field of the solar corona and solar wind (along the line of sight (LOS) between a background linearly polarized radio transmitter and a ground-based radio receiver). FR is a path integrated quantity depending on both the plasma density (ne) and the magnetic field component along the LOS (B⃗LOS). Consequently, the fundamental calibration challenge encountered when using radio FR observations to infer the magnetic field structure of the solar wind is distinguishing between the ne and B⃗LOS contributions to the detectable FR.MethodsThe method presented here, however, will be able to provide localized plasma density information to FR observations for the first time. Using a synthetic field of view of the Wide-field Imager for Parker Solar Probe (WISPR), we perform a tomographic reconstruction of the plasma density in the vicinity of spacecraft.DiscussionThis article develops the framework to determine the positions, along LOS to radio sources, of the density enhancements we reconstruct tomographically.ConclusionWhile still in development, this method provides the necessary foundation to complement and enhance coronal FR measurements, but the applicability of this technique extends beyond coronal FR.

SynCOM is a package of procedures written in IDL (Interactive Data Language) that simulates transient solar wind flows. Each function within SynCOM handles specific tasks, such as initializing parameters, generating synthetic profiles, creating Gaussian blobs to represent solar wind features, and producing high-resolution images of the solar corona. This modular design allows users to call or customize individual functions independently, providing flexibility to adjust simulations to different observational or solar wind conditions. The software architecture is designed to facilitate SynCOM, which effectively creates synthetic datasets for testing and verifying feature tracking algorithms. It also takes advantage of the robust capabilities of the IDL for high-performance scientific computing

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a four-spacecraft observatory designed for low earth orbit observations of the Sun to understand better the solar wind. Each of the four observatories carries a Teledyne e2V 230-82 CCD controlled by a Rutherford Appleton Laboratories (RAL) detector controller and a filter wheel that allows for the selection of different polarization vectors and a blank off to monitor detector health. The CCD is 2kx4k pixels and has a store shield covering half the device to serve as a charge storage region. The CCDs are operated in pseudo frame transfer mode. We present here the laboratory optical calibration data for the four flight detector systems.

We describe a novel method for modeling the global, steady solar wind using photospheric magnetic fields as a driving boundary condition. Prior wind models in this class include both rapid heuristic methods that use potential field extrapolation and variants thereof, trading rigor for computation speed, and detailed 3D magnetohydrodynamic (MHD) models that attempt to simulate the entire solar corona with a degree of physical rigor, but require large amounts of computation. The Field Line Universal relaXer, an open-source numerical code that implements the “fluxon” semi-Lagrangian approach to MHD modeling, provides an intermediate approach between these two general classes. In particular, the fluxon approach to MHD describes the magnetic field through discrete analogs of magnetic field lines, relaxing these structures to a stationary state of force balance. In this work we introduce a 1D solar wind solution along each field line, providing an ensemble of solutions that are interpolated back onto a uniform grid at an outer boundary surface. This provides advantages in physical rigor over heuristic semianalytic techniques, and in computational efficiency over full 3D MHD techniques. Here we describe the underlying methodology and the FLUXPipe modeling pipeline process.

We present the design of a portable coronagraph, CATEcor (where CATE stands for Continental-America Telescope Eclipse), that incorporates a novel “shaded-truss” style of external occultation and serves as a proof-of-concept for that family of coronagraphs. The shaded-truss design style has the potential for broad application in various scientific settings. We conceived CATEcor itself as a simple instrument to observe the corona during the darker skies available during a partial solar eclipse, or for students or interested amateurs to detect the corona under ideal noneclipsed conditions. CATEcor is therefore optimized for simplicity and accessibility to the public. It is implemented using an existing dioptric telescope and an adapter rig that mounts in front of the objective lens, restricting the telescope aperture and providing external occultation. The adapter rig, including occulter, is fabricated using fusion deposition modeling (FDM; colloquially “3D printing”), greatly reducing cost. The structure is designed to be integrated with moderate care and may be replicated in a university or amateur setting. While CATEcor is a simple demonstration unit, the design concept, process, and trades are useful for other more sophisticated coronagraphs in the same general family, which might operate under normal daytime skies outside the annular-eclipse conditions used for CATEcor.

We report on a study of the Multiview Observatory for Solar Terrestrial Science (MOST) mission that will provide comprehensive imagery and time series data needed to understand the magnetic connection between the solar interior and the solar atmosphere/inner heliosphere. MOST will build upon the successes of SOHO and STEREO missions with new views of the Sun and enhanced instrument capabilities. This article is based on a study conducted at NASA Goddard Space Flight Center that determined the required instrument refinement, spacecraft accommodation, launch configuration, and flight dynamics for mission success. MOST is envisioned as the next generation great observatory positioned to obtain three-dimensional information of large-scale heliospheric structures such as coronal mass ejections, stream interaction regions, and the solar wind itself. The MOST mission consists of 2 pairs of spacecraft located in the vicinity of Sun-Earth Lagrange points L4 (MOST1, MOST3) and L5 (MOST2 and MOST4). The spacecraft stationed at L4 (MOST1) and L5 (MOST2) will each carry seven remote-sensing and three in-situ instrument suites, including a novel radio package known as the Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH). MOST3 and MOST4 will carry only the FETCH instruments and are positioned at variable locations along the Earth orbit up to 20° ahead of L4 and 20° behind L5, respectively. FETCH will have polarized radio transmitters and receivers on all four spacecraft to measure the magnetic content of solar wind structures propagating from the Sun to Earth using the Faraday rotation technique. The MOST mission will be able to sample the magnetized plasma throughout the Sun-Earth connected space during the mission lifetime over a solar cycle.

We present results of a dual eclipse expedition to observe the solar corona from two sites during the annular solar eclipse of 14 October 2023 using a novel coronagraph designed to be accessible for amateurs and students to build and deploy. The coronagraph (CATEcor) builds on the standardized eclipse observing equipment developed for the Citizen CATE 2024 experiment. The observing sites were selected for likelihood of clear observations, for historic relevance (near the Climax site in the Colorado Rocky Mountains), and for centrality to the annular eclipse path (atop Sandia Peak above Albuquerque, New Mexico). The novel portion of CATEcor is an external occulter assembly that slips over the front of a conventional dioptric telescope, forming a shaded-truss externally occulted coronagraph. CATEcor is specifically designed to be easily constructed in a garage or “makerspace” environment. We successfully observed some bright features in the solar corona to an altitude of approximately 2.25 R⊙ during the annular phases of the eclipse. Future improvements to the design, in progress now, will reduce both stray light and image artifacts; our objective is to develop a design that can be operated successfully by amateur astronomers at sufficient altitude even without the darkened skies of a partial or annular eclipse.

We present progress on the translational tomography technique for measuring the three-dimensional structure of the corona from near-perihelion Wide-field Imager for Parker Solar Probe (WISPR) image sequences. Translational tomography makes use of noncircular motion of a camera to extract three-dimensional information from an optically thin subject. Parker Solar Probe (PSP) presents a special case both because of the particular structure of the corona and because of the nonlinear motion of the vantage point. We show improvements to a previous direct analytic method (described in Paper I of this series) and an alternative inversion pathway using a synthetic sequence of WISPR images. The newer method successfully reconstructs the correct locations of modeled coronal rays in a synthetic WISPR image sequence, with curvilinear camera motion modeled on the PSP orbit. We present the refined methodology and validation study, and show that the technique is ready for application to actual WISPR data.

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) will answer key questions about coronal mass ejections (CMEs) and the solar wind through 3D imaging made possible by properties of polarized light in the solar environment. In this work, we detail the goals of the PUNCH mission and strategy used to create 3D images. We introduce solpolpy, a Python package developed by the PUNCH Science Operations Center (SOC) to carry out this analysis. As a case study, we apply the technique to existing data and confirm the results are plausible. Finally, we outline future plans for this technique and software.

Aiming to assess the progress and current challenges on the formidable problem of the prediction of solar energetic events since the COSPAR / International Living With a Star (ILWS) Roadmap paper of Schrijver et al. (2015), we attempt an overview of the current status of global research efforts. By solar energetic events we refer to flares, coronal mass ejections (CMEs), and solar energetic particle (SEP) events. The emphasis, therefore, is on the prediction methods of solar flares and eruptions, as well as their associated SEP manifestations. This work complements the COSPAR International Space Weather Action Teams (ISWAT) review paper on the understanding of solar eruptions by Linton et al. (2023) (hereafter, ISWAT review papers are conventionally referred to as ’Cluster’ papers, given the ISWAT structure). Understanding solar flares and eruptions as instabilities occurring above the nominal background of solar activity is a core solar physics problem. We show that effectively predicting them stands on two pillars: physics and statistics. With statistical methods appearing at an increasing pace over the last 40 years, the last two decades have brought the critical realization that data science needs to be involved, as well, as volumes of diverse ground- and space-based data give rise to a Big Data landscape that cannot be handled, let alone processed, with conventional statistics. Dimensionality reduction in immense parameter spaces with the dual aim of both interpreting and forecasting solar energetic events has brought artificial intelligence (AI) methodologies, in variants of machine and deep learning, developed particularly for tackling Big Data problems. With interdisciplinarity firmly present, we outline an envisioned framework on which statistical and AI methodologies should be verified in terms of performance and validated against each other. We emphasize that a homogenized and streamlined (i.e., readily performed) method validation is another open challenge. The performance of the plethora of methods is typically far from perfect, with physical reasons to blame, besides practical shortcomings: imperfect data, data gaps and a lack of multiple, and meaningful, vantage points of solar observations. We briefly discuss these issues, too, that shape our desired short- and long-term objectives for an efficient future predictive capability. A central aim of this article is to trigger meaningful, targeted discussions that will compel the community to adopt standards for performance verification and validation, which could be maintained and enriched by institutions such as NASA’s Community Coordinated Modeling Center (CCMC) and the community-driven COSPAR/ISWAT initiative.

The Synthetic Corona Outflow Model (SynCOM), an empirical model, simulates the solar corona’s dynamics to match high-resolution observations, providing a useful resource for testing velocity measurement algorithms. SynCOM generates synthetic images depicting radial variability in total brightness and includes stochastic elements for plasma outflows and instrumental noise. It employs a predefined probability distribution for flow velocity and an adjustable signal-to-noise ratio to evaluate different data analysis methods for coronal flows. By adjusting parameters to match specific coronal and instrumental conditions, SynCOM offers a platform to assess these methods for determining coronal velocity and acceleration. Validating these measurements would help us to understand the origins of the solar wind and support missions such as the Polarimeter to Unify the Corona and Heliosphere (PUNCH). In this study, we demonstrate how SynCOM can be employed to assess the precision and performance of two different flow-tracking methods. By providing a ground truth based on observational data, we highlight the importance of SynCOM in confirming observational standards for detecting coronal flows.

The Polarimeter to UNify the Corona and Heliosphere (PUNCH) will image macroscopic features of the inner heliosphere and also admit sufficiently high spatial resolution to probe scales of turbulence within the upper end of the inertial range, close to the integral scale. As PUNCH is an imager, its measurements will relate differently to the underlying turbulent environment of the outer corona and inner heliosphere from more familiar in situ samples. We present a numerical study that combines magnetohydrodynamic simulations of turbulence together with FORWARD-modeling synthesis of white-light data via the FORWARD code. We show that (i) the "usual" turbulence scalings are modified by the integration along the LOS in an optically thin medium, and (ii) those scalings are still linked to the original properties of the turbulent field. This study is a first step in the process of analyzing and understanding the unprecedented information that PUNCH will provide.

Whitepaper #089 in the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033. Main topics: space weather applications; basic research. Additional topics: space weather research/operations/research loop; solar physics; space-based missions/projects; […]

We present results of a quantitative analysis of structured plasma outflows above a polar coronal hole observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) spacecraft. In a 6 hr interval of continuous high-cadence SDO/AIA images, we identified more than 2300 episodes of small-scale plasma flows in the polar corona. The mean upward flow speed measured by the surfing transform technique is estimated to be 122 ± 34 km s ^−1 , which is comparable to the local sound speed. The typical recurrence period of the flow episodes is 10–30 minutes, and the mean duration and transverse size of each episode are about 3–5 minutes and 3–4 Mm, respectively. The largest identifiable episodes last for tens of minutes and reach widths up to 40 Mm. For the first time, we demonstrate that the polar coronal-hole outflows obey a family of power-law probability distributions characteristic of impulsive interchange magnetic reconnection. Turbulent photospheric driving may play a crucial role in releasing magnetically confined plasma onto open field. The estimated occurrence rate of the detected self-similar coronal outflows is sufficient for them to make a dominant contribution to the fast-wind mass and energy fluxes and to account for the wind’s small-scale structure.