This visualization of the Sun's Alfvén surface, with regions below the surface shown using a total eclipse image from astronomer Miloslav Druckmüller, and regions above the surface shown in gray. The frothy/fractal shapes come from computer simulations made by PUNCH Co-I Rohit Chhiber, and they show how a given line extending away from the Sun can cross through the Alfvén surface multiple times.
PUNCH is about to get a close look at a “point-of-no-return” zone surrounding the Sun. Charged particles fly away from the Sun in the form of the steady solar wind and bursty coronal mass ejections (CMEs), and most of those particles escape all the way out past the planets of our solar system. Sometimes, though, particles fall back down, but only if they're below the Sun's Alfvén surface --- named after Nobel-Prize-winning physicist Hannes Alfvén. This surface has been theorized since the 1960s, but has only been explored directly since 2021, when NASA's Parker Solar Probe plunged through it for the first time.
PUNCH will essentially observe this zone 24/7 by measuring the motions of tiny blobs (“leaves in the wind”) that flow down toward the Sun. It will also see whether this surface is relatively smooth, like a bubble, or whether its shape is intermittent and "frothy," as illustrated above. PUNCH. In preparation for these new observations, Co-Investigator Steven Cranmer and colleagues have reviewed the current state of our understanding of the Alfvén surface as our first featured paper from the Solar Physics PUNCH Mission Overview Topical Issue, providing context and guidance for PUNCH we learn about this elusive region of space.
This is PUNCH Nugget #16. PUNCH nuggets are archived at the PUNCH mission website. You can sign up to receive PUNCH nuggets by email. NASA official releases about PUNCH are at the NASA PUNCH blog.
