The team mapped the region known as the Alfven surface, where the outward flow of the solar wind exceeds the speed at which magnetic waves can travel, creating a practical outer edge to the Sun's atmosphere. Beyond this point, escaping material cannot fall back, providing a natural laboratory for studying how solar activity shapes conditions throughout the solar system, including effects on space weather and technology near Earth.
To build and test the maps, scientists used NASA's Parker Solar Probe, which repeatedly flies deep into the Sun's atmosphere, together with measurements from near-Earth spacecraft. The study shows that as the Sun moves toward the active phase of its roughly 11-year cycle, the Alfven surface grows in size, becomes rougher, and develops pronounced spikes.
"Parker Solar Probe data from deep below the Alfven surface could help answer big questions about the Sun's corona, like why it's so hot. But to answer those questions, we first need to know exactly where the boundary is," said Sam Badman, an astrophysicist at the CfA and the paper's lead author. The team directly validated the new maps using Parker's closest passes through the Sun's atmosphere and reported the results in Astrophysical Journal Letters.
Using Parker's Solar Wind Electrons Alphas and Protons (SWEAP) instrument, developed at the CfA with the University of California, Berkeley, researchers collected data from deep within the sub-Alfvenic region near the Sun. Michael Stevens, a CfA astronomer and principal investigator for SWEAP, said, "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. We are now headed for an exciting period where it will witness firsthand how those processes change as the Sun goes into the next phase of its activity cycle."
Badman noted that until now scientists could only estimate the Sun's outer boundary from afar with limited ways to verify those estimates. "Before, we could only estimate the sun's boundary from far away without a way to test if we got the right answer, but now we have an accurate map that we can use to navigate it as we study it," Badman said, adding that repeated mapping allows the team to track boundary changes and compare them directly with close-up measurements.
Earlier work suggested that the Alfven surface shifts with the solar cycle, moving outward and becoming more structured and complex during solar maximum and shrinking and smoothing during solar minimum. The new maps confirm that as solar activity rises, the shape and height of this boundary increase and become more sharply structured, matching previous predictions.
The data set provides new constraints on the physics operating deep in the corona and can feed into improved solar wind and space weather models. These enhanced models should refine forecasts of how solar eruptions and changing solar wind conditions propagate through interplanetary space and affect planetary environments.
The results also inform studies of other stars, where similar processes may govern how stellar winds evolve and interact with orbiting planets. Understanding how the Sun's boundary changes over time can help researchers assess how stellar activity influences planetary atmospheres and potential habitability across the galaxy.
The project relied on a coordinated multi-spacecraft strategy that combined Parker's close passes with more distant observations from missions such as Solar Orbiter, a joint NASA-ESA project, and NASA's Wind spacecraft. The team plans to repeat the campaign during the next solar minimum to capture how the Alfven surface evolves over an entire solar cycle.
Research Report:Multi-spacecraft measurements of the evolving geometry of the Solar Alfven surface over half a solar cycle
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