Scientists have created the first detailed maps of the outer edge of the sun’s atmosphere, a shifting boundary where solar material breaks free of the sun’s magnetic grip and streams out into space.
The new maps, built using close-up measurements from NASA’s Parker Solar Probe along with data from more distant spacecraft, show that this boundary grows larger, rougher and more jagged as the sun becomes more active, periods in its cycle that are marked by increased sunspots and solar flares.
“Before, we could only estimate the sun’s boundary from far away without a way to test if we got the right answer,” study lead author Sam Badman, an astrophysicist at the Center for Astrophysics ∣ Harvard & Smithsonian (CfA) in Massachusetts, said in a statement. “But now we have an accurate map that we can use to navigate it as we study it.”
“And, importantly, we also are able to watch it as it changes and match those changes with close-up data,” he added. “That gives us a much clearer idea of what’s really happening around the sun.”
The boundary, known as the Alfvén surface, marks the point where the outward flow of the solar wind becomes faster than magnetic waves that would otherwise carry material back toward the sun. Beyond this “point of no return,” solar particles can no longer fall back and instead stream permanently into interplanetary space.
Scientists knew that this boundary shifts with the sun’s roughly 11-year activity cycle — expanding and becoming more complex during solar maximum, and shrinking during quieter solar minimum periods. Until now, however, they lacked direct confirmation of what those changes actually looked like.
“That’s actually what we predicted in the past, but now we can confirm it directly,” Badman said in the statement.
To build the new maps, the researchers combined close-up measurements from the Parker Solar Probe, which repeatedly plunged through the sun’s outer atmosphere during record-breaking close passes as the solar cycle ramped up toward its peak, with data from the European Space Agency‘s Solar Orbiter and NASA’s Wind mission, both of which reside about 1 million miles (1.5 million kilometers) from Earth.
Using an instrument onboard Parker called the Solar Wind Electrons Alphas and Protons (SWEAP), the team directly sampled the region beneath the Alfvén surface, confirming that the maps correctly show where the sun’s magnetic influence fades and the solar wind escapes, according to the statement.
“This work shows without a doubt that Parker Solar Probe is diving deep with every orbit into the region where the solar wind is born,” study co-author Michael Stevens, an astronomer at the CfA and the principal investigator of the SWEAP instrument, said in the statement.
Pinpointing where and how the solar wind escapes the sun is essential to answering some of the biggest open questions in solar physics, including why the sun’s corona gets hotter the farther it extends away from the solar surface.
Understanding exactly where this boundary lies is also critical for improving space weather forecasts, which help protect astronauts in space, and satellites and power grids on Earth from disruptive solar storms, scientists say.
During the next solar minimum, the Parker Solar Probe will again plunge deep into the sun’s atmosphere, allowing scientists to watch how this boundary evolves over a complete solar cycle, according to the statement.
“There are still a number of fascinating physics questions about the sun’s corona that we don’t fully understand,” said Stevens.
