The sun’s so-called “slow” solar wind has been caught speeding. Observations from ESA’s Proba-3 mission, published in The Astrophysical Journal Letters in April 2026, show that plasma streaming outward through the inner corona moves three to four times faster than models predicted, upending decades of assumptions about where and how the solar wind picks up speed.
“We expected to see sluggish outflows in this region, and instead we found gusts that were several times faster than any model had forecast,” said Andrei Zhukov, principal investigator for the mission’s ASPIICS coronagraph at the Royal Observatory of Belgium and lead author of the study, in an ESA announcement.
The result matters beyond heliophysics. Space-weather forecasting models, which help protect satellites, astronaut safety, and power grids on Earth, rely on assumptions about how quickly solar wind accelerates away from the sun. If those assumptions are significantly wrong in the inner corona, the models’ predictions about when and how intensely a solar storm will hit Earth could be off as well.
How Proba-3 pulled it off
Proba-3 is not one spacecraft but two, flying in precise formation. One satellite acts as a disk that blocks the blinding solar surface for the other, which carries the ASPIICS coronagraph. The arrangement mimics a total solar eclipse on demand, with advanced stray-light suppression that lets the instrument image the corona at high spatial and temporal resolution between roughly 1.3 and 3 solar radii from the sun’s center.
That zone has been notoriously difficult to observe continuously. Ground-based coronagraphs contend with Earth’s atmosphere; single-spacecraft instruments struggle with scattered light so close to the solar disk. Proba-3’s formation-flying design was built specifically to fill this gap, and the first science results suggest the investment paid off. Within that narrow band, ASPIICS detected persistent outflows and inflows moving far faster than the textbook picture of slow wind, which is typically characterized by speeds of roughly 300 to 400 kilometers per second when measured much farther from the sun, near Earth’s orbit.
Independent evidence lines up
Proba-3 is not operating in a vacuum. Two other independent measurement techniques point in the same direction, making it harder to dismiss the finding as an instrument quirk.
Solar Orbiter’s Metis coronagraph, which observes in ultraviolet light, has separately measured slow-wind velocities and acceleration behavior in the inner-to-mid corona. Those data show that even streams classified as “slow” are already traveling at substantial fractions of their eventual speed just a few solar radii out.
Meanwhile, a peer-reviewed study in Monthly Notices of the Royal Astronomical Society used simultaneous radio-science experiments from the BepiColombo and Akatsuki missions, cross-correlating density fluctuations in spacecraft radio links to pin down near-sun wind speeds with explicit velocity estimates and uncertainties.
Three distinct techniques, each sensitive to different sources of error, all converge on the same conclusion: the slow wind does not start slow and gradually creep up to speed. It is energized relatively close to the sun and then largely coasts outward. That convergence is what gives heliophysicists confidence the signal is real.
What remains uncertain
Confidence in the broad finding does not mean every detail is settled. The exact velocity figures and error bars from ASPIICS have not yet been independently reproduced by a separate team using the same dataset. The qualitative conclusion of “faster than expected” appears robust, but the precise “three to four times” multiplier could shift as more data are analyzed.
It is also unclear how universal the result is. The fast acceleration may be typical of certain coronal structures, such as helmet streamers, but less pronounced in pseudostreamers or other slow-wind source regions. Longer observing campaigns spanning multiple solar rotations will be needed to determine whether the early speed-up is a constant background feature or dominated by intermittent gusts tied to specific phases of the solar cycle.
The mission itself hit a snag that adds a practical wrinkle. ESA disclosed that controllers lost and later restored contact with the coronagraph spacecraft before the science announcement. After recovery, the instrument was operated in manual mode for the first time, according to a Proba-3 mission blog update dated April 3, 2026. Public updates have not specified how much observing time was lost or whether the key data were collected before, after, or across the outage. That ambiguity does not undermine the findings, but it does mean analysts will want clear documentation of how manual-mode operations compare to the nominal configuration.
Perhaps the biggest open question is physical: what drives the unexpectedly fast acceleration? One hypothesis points to magnetic reconnection events in the low corona injecting energy into the slow wind earlier than magnetohydrodynamic models assume. Another invokes Alfvén waves depositing more energy at low heights than previously modeled. Distinguishing between these ideas will require cross-correlating ASPIICS flow patterns with magnetic-field maps and density structures from Solar Orbiter over multiple solar rotations, a coordinated campaign that has not yet been reported.
Where Parker Solar Probe fits in
Readers familiar with solar-wind research may wonder about NASA’s Parker Solar Probe, which has flown closer to the sun than any spacecraft in history. Parker measures the solar wind in situ, sampling particles and magnetic fields directly, but its closest passes cut through the corona at altitudes that overlap only partially with the 1.3-to-3 solar radii zone where ASPIICS detected the fast outflows. Parker’s data have already revealed surprising complexity in near-sun wind behavior, including magnetic switchbacks and bursts of fast plasma. Combining Parker’s in-situ measurements with Proba-3’s remote imaging of the same region could eventually provide a far more complete picture of how the slow wind is born and accelerated.
What this means for space-weather forecasting
If the slow wind accelerates much closer to the sun than current models assume, forecasters will need to update the physics baked into their simulations. Most operational space-weather models treat the inner corona as a zone of gradual, smooth acceleration. A steeper, more abrupt speed-up would change predictions about when coronal mass ejections and solar-wind streams arrive at Earth, how they interact with Earth’s magnetosphere, and how much lead time satellite operators and power-grid managers can count on.
As Proba-3 continues operations and joint campaigns with Solar Orbiter and other missions ramp up through 2026, the gaps in the data should narrow. For now, the “slow” solar wind has earned a new reputation: close to its source, it is anything but leisurely.
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*This article was researched with the help of AI, with human editors creating the final content.
