3D Space Weather Imaging with PUNCH (NASA's Polarimeter to Unify the Corona and Heliosphere) (Invited Presentation)

The Polarimeter to UNify the Corona and Heliosphere is a Small Explorer mission selected by NASA to understand the Sun's outer corona and the solar wind as a single unified system. PUNCH will launch in 2023, comprises four satellites in twilight Sun-synchronous low Earth orbit, and will produce low-noise images of the entire outer corona and inner heliosphere every four minutes. The science objectives are (1) to understand how coronal structures become the ambient solar wind, and (2) to understand the dynamic evolution of transient structures (e.g., CMEs) in the young solar wind. PUNCH will return unique 3D information about CME structure, chirality, and propagation, to advance the science of space weather. Mission enhancement options under study include a real-time enhancement that would enable direct prediction of space weather through rapid, low-latency downlink of PUNCH images.

The PUNCH instruments are a single Narrow-Field Imager (NFI) implemented as a compact externally-occulted coronagraph built by the Naval Research Laboratory, and three identical Wide-Field Imagers (WFIs) implemented as straight-baffled corral-type heliospheric imagers. The instruments are matched in exposure time and spectral response, overlap in their fields of view, and are operated synchronously. The result is a single "virtual instrument" with an integrated field of view spanning all position angles around the Sun, from 1.25° (5 apparent solar radii) to 45° (180 apparent solar radii) from Sun center. The entire suite acquires one polarized image sequence every four minutes for a nominal mission of two years. The data are merged on the ground into integrated, background-subtracted, polarized (B & pB) data products at 4 minute and 32 minute cadence.

PUNCH sensitivity is specified over 1° square patches of sky, relative to the mean photospheric radiance (Bsun). Compared to contemporary imagers, PUNCH is 10x to 30x more sensitive, with 5x-25x higher cadence and full coverage of position angles. It is 1.2E-15 Bsun at 15 Rs, and 4E-17 Bsun at 80 Rs, to ensure photometry and 3D location of solar wind features and CMEs, via the polarization ratio method, throughout the field of view.

We will describe the PUNCH measurements and how they will potentially revolutionize the science of space weather prediction by tracking both the "background" solar wind and geoeffective CMEs in 3D as they cross the interplanetary void.