First maps of the sun’s outer boundary could help predict solar storms

The sun’s outer atmosphere resembles a puffer fish.

That’s what scientists have gleaned from the first verified maps of the shifting boundary between the sun and the rest of the solar system. “The structure basically has a wavy, spiky shape,” says heliophysicist Sam Badman. As the sun becomes more active, the border becomes bigger and thornierBadman and colleagues report in the Dec. 20 Letters from astrophysical journals.

Maps of this boundary, known as the Alfvén critical surface, could help scientists better predict how solar activity affects satellites, human and animal health and atmospheric phenomena such as the Northern Lights.

Alfvén’s invisible surface marks a point of no return for plasma and particles leaving the sun to become the solar wind. It roughly demarcates “the boundary between the corona, which is what you see during an eclipse, and the solar wind,” says Badman, of the Harvard-Smithsonian Center for Astrophysics.

Previously, scientists estimated the size and shape of Alfvén’s surface using observations from spacecraft located about as far from the sun as Earth. But in 2021, NASA’s Parker solar probe ventured beneath the surface of Alfvén for the first time when it rose about 13 million kilometers from the surface of the sun. Since then, the spacecraft has plunged into the corona 15 more times, diving less than 6.1 million kilometers of the sun in December 2024.

Badman and his colleagues combined Parker’s direct measurements of Alfvén’s surface with remote data from other solar-observing spacecraft to create detailed maps of the shape of the boundary and the density, speed and temperature of the plasma there.

The team also tracked how the surface evolved during half of the Sun’s 11-year magnetic cycle. As the sun moved from solar minimum, characterized by relatively few sunspots and solar flares, to solar maximum, Alfvén’s surface expanded and became messier and less spherical. Researchers hope to observe Parker repeatedly diving into the surface as the sun returns to solar minimum over the next five years.

Understanding the Alfvén surface of the Sun could have implications for planets outside the solar system. Some planets orbit right next to their starssome of which are much more magnetically active than the sun, leading to Alfvén surfaces that extend further. A nearby planet orbiting a sufficiently active star could spend its entire life inside the Alfvén boundary.

“It’s probably not going to be good news for livability,” Badman says.