Polarimeter to Unify the Corona and Heliosphere (PUNCH)

TL;DR

PUNCH tackles the longstanding challenge of linking the solar corona to the inner heliosphere by delivering globally distributed, polarization-sensitive imaging of Thomson-scattered light from the K corona and young solar wind. The mission deploys a 1+3 constellation of small spacecraft that operate as a single virtual instrument, providing a 90° field of view and enabling 3D localization of structures such as CMEs, turbulence mesostructures, and the Alfvén surface. By combining deep polarimetric sequences with synchronized, multi-scale observations, PUNCH aims to quantify wind flow, variability, and cross-scale dynamics across the inner solar system, and to track transients in three dimensions. The data products and open data policy further enhance the mission’s impact, extending its utility to stellar, planetary, solar-system dust, and geospace studies beyond the primary science goals.

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 degrees (6 R$_\odot$) to 8 degrees (32 R$_\odot$); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3 degrees (12 R$_\odot$) to 45 degrees (180 R$_\odot$) 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 degree 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.

Figures (19)

• Figure 1: Artist's rendering shows the six major PUNCH scientific topics, each addressed by a science question. These are organized into two major science objectives (understanding the ambient and the dynamic solar wind) in support of a single science goal: to determine the cross-scale processes that unite the solar corona and heliosphere.
• Figure 2: Four PUNCH spacecraft view a 90$^\circ$ wide field of view on the celestial sphere, centered on the Sun. One Narrow Field Imager (NFI) captures the outer solar corona to an elongation of 32 R$_\odot$. Three Wide Field Imagers (WFIs) together capture a trefoil instantaneous field of view (IFOV) on the sky. The trefoil figure rotates once per orbit, capturing the complete field of view (FOV; yellow circle) approximately once every 30 minutes. The fields of view overlap at elongations below 80 R$_\odot$, providing 4 minute cadence in the inner part of the FOV.
• Figure 3: STEREO COR2 white light coronagraph observations from a deep-field campaign deforest_etal_2018, with 72 hours of long exposures at 5-minute cadence, reveal outer coronal structure. Left image has had smooth background and stars removed, showing the outer corona dominated by fine "woodgrain" structure. Outflow is visible everywhere because of small moving features, and further processing (right image) shows a riotous torrent of blobs and variable streams, tracing flow of the young solar wind.
• Figure 4: PUNCH eliminates observational gaps between coronal and heliospheric imagers and at the poles, continuously observing the young solar wind with a global field of view (FOV). Shown are STEREO COR2 and HI FOVs, within the full PUNCH FOV.
• Figure 5: PUNCH is designed to reveal the development of turbulence in the solar wind. Observations of white-light brightness from STEREO/SECCHI reveal spectral steepening with altitude, suggesting onset of turbulence. With 10 times higher sensitivity, PUNCH is designed to resolve the hinted inertial range at spatial frequencies up to 5 Gm$^{-1}$ (200 Mm scale) and to altitudes of 150 Gm.