Sun’s jagged outer boundary tracked: Where solar wind breaks free

Solar Flare (Image Credit - Canva)

Scientists have produced the first detailed maps of the sun’s outer atmosphere, revealing the shifting boundary where solar material escapes into space. The research, published Dec. 11 in The Astrophysical Journal Letters, combines close-up data from NASA’s Parker Solar Probe with measurements from other spacecraft.

The maps show the Alfvén surface, the point where solar wind surpasses magnetic waves. Beyond this boundary, particles escape permanently, no longer returning to the sun. Researchers found that the surface grows larger and more jagged as the sun becomes more active, during periods of increased sunspots and solar flares.

Before these observations, scientists could only estimate the sun’s boundary from afar. "Now we have an accurate map that we can use to navigate it," said Sam Badman, lead author and astrophysicist at the Center for Astrophysics ∣ Harvard & Smithsonian. He added that observing changes in real time offers a clearer view of solar behaviour.

An artist’s rendering depicts the sun’s outer atmospheric boundary. NASA’s Parker Solar Probe and other spacecraft data reveal it becomes “larger, rougher, and more jagged” during increased solar activity. (Image: CfA/ Melissa Weiss)

The Parker Solar Probe repeatedly plunged into the sun’s outer atmosphere, gathering close-up measurements as solar activity approached its peak. These data were combined with observations from the European Space Agency’s Solar Orbiter and NASA’s Wind mission, orbiting about 1 million miles from Earth.

Using the SWEAP instrument onboard Parker, the team directly sampled the region beneath the Alfvén surface. This confirmed the maps accurately represent where the sun’s magnetic influence fades and the solar wind escapes, explained Michael Stevens, SWEAP principal investigator and astronomer at the CfA.

Pinpointing where the solar wind originates helps answer major questions in solar physics, including why the corona heats as it extends outward. Understanding this boundary also improves space weather forecasts, vital for protecting astronauts, satellites, and Earth’s power grids from solar storms.

The Parker Solar Probe will continue its mission through the next solar minimum, offering scientists the chance to observe the Alfvén surface across a full solar cycle. "There remain many fascinating questions about the sun’s corona that we do not fully understand," Stevens said.