Astronomers map the Sun’s true outer edge for the first time

Astronomers have produced the first continuous two-dimensional maps of the Sun’s outer atmospheric boundary, revealing a complex and evolving structure that defines where the solar atmosphere truly ends.

This boundary marks the location where solar material escapes the Sun’s magnetic control and flows freely into interplanetary space, forming the solar wind that fills the solar system.

The achievement was made possible by combining close-range measurements from NASA’s Parker Solar Probe with observations from spacecraft positioned farther from the Sun, allowing direct validation of long-standing theoretical predictions.

The work was led by scientists at the Center for Astrophysics, Harvard and Smithsonian, and published in The Astrophysical Journal Letters on December 11.

Together, these observations transform the Alfvén surface from a theoretical construct into a directly measured and mapped feature of the Sun.

A boundary that controls solar wind escape

The Alfvén surface marks the point in the Sun’s atmosphere where the outward speed of the solar wind exceeds the speed of magnetic waves traveling through the plasma.

Below this surface, the Sun’s magnetic field remains dominant, allowing disturbances and energy to propagate both toward and away from the Sun.

Above it, the solar wind becomes magnetically disconnected, meaning information and energy can no longer travel back toward the Sun.

For this reason, scientists describe the Alfvén surface as the point of no return for solar material, since plasma crossing it is irreversibly lost to interplanetary space.

Although the visible corona extends millions of kilometers beyond this boundary, the Alfvén surface defines the Sun’s effective physical edge in terms of magnetic influence.

Parker Solar Probe dives into uncharted territory

The confirmation of the Alfvén surface was made possible by NASA’s Parker Solar Probe, the first spacecraft designed to repeatedly pass through the Sun’s outer atmosphere.

A key role was played by the Solar Wind Electrons Alphas and Protons (SWEAP) instrument, developed by the Center for Astrophysics in collaboration with the University of California, Berkeley.

SWEAP measures the speed, density, and temperature of solar wind particles, allowing scientists to determine whether the plasma is moving slower or faster than local magnetic waves.

Parker first crossed the Alfvén surface in 2021 and has since made repeated deep dives below it, providing the ground truth needed to validate large-scale models.

These measurements confirm that Parker is now routinely sampling the region where the solar wind is born.

Building the first continuous two-dimensional maps

While Parker provides close-up measurements, it cannot observe the entire boundary at once.

To overcome this, researchers combined Parker data with observations from the Solar Orbiter mission and spacecraft positioned near the Sun-Earth L1 point.

This multi-spacecraft approach allowed scientists to track how the height and shape of the Alfvén surface vary with longitude and time.

By scaling solar wind properties measured far from the Sun and validating them against Parker’s direct crossings, the team produced continuous two-dimensional equatorial maps.

These maps represent the first validated depiction of a region that had previously existed only in theoretical models.

A surface shaped by the solar cycle

The maps show that the Alfvén surface is not fixed, but evolves with the Sun’s activity cycle.

During periods of low solar activity, the boundary sits closer to the Sun and appears relatively smooth and spherical.

As solar activity increases toward solar maximum, the surface expands outward and becomes thicker, rougher, and more irregular.

Over the observed portion of solar cycle 25, the average height of the Alfvén surface increased by up to about 30 percent, shifting from roughly 12 to 17 solar radii to around 15 to 23 solar radii.

This change corresponds to distances of approximately 8.3–11.8 million km (5.2–7.3 million miles) at solar minimum and up to about 10.4–16 million km (6.5 –10 million miles) near solar maximum.

Why small changes have large consequences

Although the increase in the Alfvén surface height may appear modest, its physical impact is substantial.

Angular momentum loss from the Sun depends on the square of the Alfvén surface distance, meaning even moderate outward shifts can dramatically increase the rate at which the Sun spins down.

The study shows that a roughly 30% increase in boundary height corresponds to nearly a doubling of angular momentum loss per unit mass.

This process influences how the Sun evolves over billions of years and provides a benchmark for understanding stellar rotation across the galaxy and shows the importance of accounting for solar cycle effects when modeling stellar winds and magnetic braking.

Implications for space weather near Earth

Accurately locating the Alfvén surface improves models of how solar wind structures form and propagate through the solar system.

This has direct implications for space weather forecasting, including predictions of geomagnetic storms that can disrupt satellites, navigation systems, power grids, and astronaut safety.

The maps also show that transient events such as coronal mass ejections can locally distort the Alfvén surface, creating outward protrusions that persist longer than previously expected.

Understanding these distortions helps explain why space weather conditions can remain disturbed even after an eruption has passed.

As models incorporate these findings, forecasts of solar-driven disturbances near Earth are expected to become more reliable.

What this reveals about other stars

The Sun is the only star whose Alfvén surface can be measured directly, making it a crucial reference point for stellar physics.

More active stars with stronger magnetic fields are expected to have Alfvén surfaces extending much farther into space, in some cases enveloping the orbits of close-in exoplanets.

Planets orbiting within these regions experience stronger magnetic interactions with their host stars, which can influence atmospheric loss and long-term habitability.

The newly mapped solar boundary provides a real-world benchmark for testing and calibrating stellar wind models applied to distant stars. In this way, understanding the Sun’s edge helps scientists interpret the environments of planetary systems across the galaxy.

A foundation for future solar exploration

The research team plans to continue tracking the Alfvén surface as solar activity declines toward the next solar minimum.

By observing how the boundary contracts and smooths during quieter conditions, scientists aim to capture its full evolution over an entire solar cycle.

Future observations from Parker Solar Probe and Solar Orbiter will also allow researchers to extend these maps beyond the solar equatorial plane.

With the Sun’s outer boundary now mapped and measurable, heliophysics enters a new phase where one of the most fundamental features of our star can be studied directly rather than inferred.

References:

¹ Astronomers Create First Map of the Sun’s Outer Boundary – Harvard University – December 10, 2025

² Multispacecraft Measurements of the Evolving Geometry of the Solar Alfvén Surface over Half a Solar Cycle – Samuel T. Badman et al. – December 11, 2025 – https://iopscience.iop.org/article/10.3847/2041-8213/ae0e5c – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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