Wild New Map Shows the Sun’s ‘Point of No Return’

Stars are lively, often unpredictable parts of the universe, and many aspects of their physics remain poorly understood. That includes our own Sun, whose fickle weather patterns regularly tamper with Earth’s magnetic fields. But a new, first-of-its-kind map of the solar boundary may hint at some answers to the Sun’s many mysteries.

Astronomers led by the Center for Astrophysics | Harvard & Smithsonian (CfA) created the first continuous 2D maps of the Sun’s outer surface—a comprehensive picture of a “spiky, frothy” world fraught with solar winds, plasma, magnetic waves, and other stellar phenomena. The map, announced in a paper published today in The Astrophysical Journal Letters, was compiled from Parker Solar Probe’s data on the Sun’s surface and later refined using additional close-up measurements.

“Before, we could only estimate the Sun’s boundary from far away without a way to test if we got the right answer,” said Sam Badman, study lead author and a CfA astrophysicist, in a statement. “But now we have an accurate map that we can use to navigate it as we study it.”

Beyond the point of no return

At a certain point in the Sun’s atmosphere, the solar wind accelerates beyond the speed at which magnetic waves can travel through it. This checkpoint, called the Alfvén surface, is a “point of no return” for any material—solar winds, plasma bursts—that escapes the Sun to enter the vacuum of space.

Typically, solar stuff becomes a tangible threat to Earth once it escapes the Sun and travels toward our planet. And so, as the effective “edge” of the solar atmosphere, the Alfvén surface has been of particular interest to astrophysicists in understanding “fluctuations and turbulence in the dynamics and evolution of the corona,” according to the paper.

Mapping the singularity

NASA’s Parker Solar Probe has been routinely measuring the solar Alfvén surface since 2021—exactly the data the team needed to verify their map, which used measurements from previous spacecraft such as Solar Orbiter and the Deep Space Climate Observatory.

“Parker Solar Probe data from deep below the Alfvén surface could help answer big questions about the Sun’s corona, like why it’s so hot,” Badman added. “We are now headed for an exciting period where [the probe] will witness firsthand how those processes change as the Sun goes into the next phase of its activity cycle.”

Tracking the solar cycle

Until now, astrophysicists had a rough idea of how this boundary moved back and forth alongside solar cycles but weren’t able to confirm how that really played out. The new map changes that.

“As the Sun goes through activity cycles, what we’re seeing is that the shape and height of the Alfvén surface around the Sun is getting larger and also spikier,” Badman explained. “That’s actually what we predicted in the past, but now we can confirm it directly.”

Now, the researchers are waiting for the solar minimum, expected to arrive around 2036. But they also believe that the same model could be used to investigate the atmospheric dynamics of stars elsewhere, “from how they’re born to how they behave throughout their lives,” the researchers noted, “including how that behavior influences the habitability of their orbiting planets.”