Spiky Barrier: Astronomers chart the outer edge of the solar atmosphere for the first time—thanks to daring flights by NASA's Parker Solar Probe

NASA's Parker Probe Maps the Sun's Mysterious Alfvén Surface for the First Time

December 17, 2025

Solar System

For the first time, astronomers have mapped the outer boundary of the Sun's atmosphere—thanks to daring flights by NASA's Parker Solar Probe deep into the solar corona. Their data reveal that this so-called Alfvén surface is covered in vast, shifting spikes and expands over the course of the solar cycle. The first precise mapping of the corona's edge could help answer long-standing questions about the corona and the solar wind, the research team explains.

Extremely Hot

Filled with deadly radiation and the source of energetic particle streams, the Sun's atmosphere—the corona—is an extreme yet critically important region. It is here that the solar wind originates, its high-energy particles shaping the entire solar system. But how the corona is structured, what processes unfold within it, and why remain only partially understood. One key reason: the solar corona reaches temperatures of millions of degrees—far too hot for any spacecraft to endure.

A Probe Through the Corona

That changed with NASA's Parker Solar Probe. Since 2018, it has repeatedly plunged into the corona, coming closer to the Sun than any mission before it. This gives astronomers an unprecedented opportunity to map the corona's outer boundary in detail. Known as the Alfvén surface, this boundary marks the outer edge of the solar atmosphere and the transition from the corona into interplanetary space—a point of no return for solar wind particles.

"Previously, we could only estimate the shape and position of this solar boundary without direct on-site measurements," explain Samuel Badman of the Harvard & Smithsonian Center for Astrophysics and his colleagues. But over the past few years, the Parker Solar Probe has repeatedly flown through this boundary region, providing the first direct measurements of the Alfvén surface. It lies where the solar wind accelerates beyond the speed of magnetic waves in the Sun's magnetic field.

The Corona's Edge Expands

This has now allowed Badman and his team to map the Sun's Alfvén surface for the first time. They analyzed data from the Parker Solar Probe, combining it with observations from solar observatories and the European Space Agency's Solar Orbiter. This enabled them to determine the position and shape of the Alfvén surface during the first half of the current solar cycle. "Crucially, we can see how this solar boundary changes over time," says Badman.

The mapping reveals an outer solar boundary that resembles the spiky shell of a chestnut more than a smooth sphere. On average, the Alfvén surface lies between 12 and 23 solar radii from the Sun's surface. However, it shifts over the solar cycle: "The average height of the surface increases as the solar cycle progresses," the astronomers report. The boundary expands further as the Sun approaches solar maximum—by up to 30 percent overall.

A Spiky Ball, Not a Perfect Sphere

But that's not all: as the Sun nears its peak activity, its corona's outer edge becomes increasingly jagged and asymmetrical. "The shape of the Alfvén surface grows significantly spikier on the way to solar maximum," Badman and his team report. "Large, sudden protrusions in the boundary also become more frequent."

The Parker Solar Probe's measurements also show a direct link between these spikes, the height of the corona's edge, and sunspots. From mid-2023, the spiky shape of the Alfvén surface temporarily flattened for several months. "This coincided with a period of slightly reduced sunspot numbers," the astronomers note. This confirms the theory that the corona's outer boundary is strongly influenced by the local shape and intensity of the Sun's magnetic field.

"An Exciting Era"

This new mapping will help verify and refine the theoretical models of solar physics. Additionally, the fresh data could shed light on unresolved questions about the processes unfolding in the sun's atmosphere. "The data collected by the Parker Solar Probe deep below the Alfvén surface, for example, may help explain why the solar corona is so hot," says Badman. Models of the solar wind and forecasts of solar storms could also become more precise with this information.

"There are still some fascinating questions about the sun that we don't yet have complete answers to," explains Badman's colleague Michael Stevens. "Thanks to the Parker Solar Probe, we've now entered an exciting era where we can directly observe many of these processes and how they evolve over the solar cycle."