The fast solar wind is one of the most persistent mysteries in solar physics. It streams continuously from regions known as coronal holes, where the Sun’s magnetic field opens into space rather than looping back into the solar surface. These open magnetic field lines create channels through which plasma can escape. Yet the mechanism that supplies enough energy to heat this plasma and propel it outward remains uncertain. For decades, scientists have proposed that Alfvénic waves—oscillations involving both plasma motion and magnetic-field fluctuations—could carry energy from the lower solar atmosphere into the corona.

The new study, led by Prof. Hui Tian and Dr. Yuhang Gao of Peking University, focuses on a part of this process that has remained particularly elusive: high-frequency transverse waves. In these waves, plasma moves sideways relative to the direction of propagation, while the magnetic field oscillates along with it. Because their periods are short and their structures can be extremely narrow, such waves are easily blurred or averaged out by instruments with limited resolution. Previous observations generally detected longer-period motions, leaving open the question of whether a substantial high-frequency wave population exists.

To search for these signals, the researchers examined observations obtained by Solar Orbiter’s Extreme Ultraviolet Imager on September 14, 2021. The spacecraft was observing the Sun’s north polar coronal hole, an environment filled with bright, elongated structures called coronal plumes. These plumes are formed by concentrated plasma and magnetic-field structures and can act as natural guides for disturbances traveling through the corona. Solar Orbiter’s imaging system recorded the region at five-second intervals, with a pixel scale of approximately 210 kilometers, allowing the researchers to follow changes that would be invisible or poorly resolved in many earlier datasets.

The team transformed the image sequence into time–distance maps, a technique that reveals moving features as slanted tracks across space and time. They then used an automated wave-tracking method to identify transverse disturbances and measure their speeds, periods, and propagation behavior. The analysis uncovered 2,318 propagating wave events in the Solar Orbiter observations. The waves appeared as repeated sideways displacements of plume structures, indicating that the corona was filled with coordinated motions rather than isolated, random fluctuations.

The contrast became striking when the researchers analyzed simultaneous observations from NASA’s Solar Dynamics Observatory, whose Atmospheric Imaging Assembly has a lower spatial and temporal resolution for this purpose. Applying the same analysis to those data produced only 560 identifiable wave events. The discrepancy was greatest for the shortest periods. Waves with periods below 100 seconds represented 38 percent of the Solar Orbiter detections, but only 9 percent of the events found in the Solar Dynamics Observatory observations. This suggests that many high-frequency waves have not been absent from earlier observations; they have simply been hidden by limited resolution and cadence.

The researchers also investigated whether the newly revealed waves carried meaningful amounts of energy. Their power-spectrum analysis showed that the Solar Orbiter data contained substantially stronger wave power above approximately 10 millihertz. In the 10–30 millihertz band, the wave power was more than twice that measured in the lower-frequency range of 2–10 millihertz. When the team estimated the associated energy flux—the rate at which wave energy passes through a given area—the value derived from Solar Orbiter was about 2.6 times higher than the estimate based on the Solar Dynamics Observatory data.

That result matters because high-frequency waves can dissipate their energy more efficiently under several theoretical models. As they travel outward, interactions with the structured and magnetized coronal plasma may convert wave energy into thermal energy or bulk motion. Processes such as turbulent cascade, resonant absorption, phase mixing, and ion–cyclotron interactions have all been proposed as possible routes by which Alfvénic waves could heat the corona or transfer momentum to the solar wind. The observations do not identify which mechanism dominates, but they demonstrate that the high-frequency portion of the wave spectrum may contain far more energy than previously recognized.

The study also highlights why the Sun’s polar regions are so important—and so difficult to observe. Telescopes positioned near Earth’s orbital plane view the polar corona from an oblique angle, causing multiple plume structures to overlap along the line of sight. That projection effect can obscure the location where waves originate and make their direction of travel difficult to determine. Future missions designed to obtain a more direct view of the poles, including China’s planned Solar Polar-orbit Observatory, could combine ultraviolet imaging with magnetic-field measurements and in-situ sampling of the solar wind. Such observations may reveal how these waves are generated, how they evolve as they move through the corona, and whether they provide the energy required to launch the fast solar wind into interplanetary space.

Subject of Research: High-frequency transverse magnetohydrodynamic waves in coronal plumes and their possible role in coronal heating and fast solar-wind acceleration.

Web References: https://doi.org/10.1093/nsr/nwag370

References: National Science Review, DOI: 10.1093/nsr/nwag370

Image Credits: © Science China Press

Keywords

Solar Orbiter, fast solar wind, Alfvénic waves, magnetohydrodynamic waves, coronal plumes, polar corona, coronal holes, solar physics, coronal heating, Solar Dynamics Observatory

The phenomenon, described by the team as “super expansion,” was detected as the magnetic cloud traveled the relatively short expanse of approximately 13 million miles between the two spacecraft at distances of 0.84 and 0.98 astronomical units (AU) from the sun. During this interval, the cloud swelled by about 21% beyond its original dimension while plasma inside the bubble experienced dramatic heating, reaching temperatures three times greater than typical prior to expansion. This observation challenges long-held assumptions about magnetic cloud evolution in interplanetary space, revealing dynamic processes not fully accounted for in current models.

Magnetic clouds are dense, magnetized plasma structures frequently generated during CMEs—solar eruptions that hurl vast amounts of energetic material and magnetic fields into the solar system. When these clouds are Earth-directed, they pose serious risks to technological infrastructure by disrupting satellites, communication systems, and power grids through geomagnetic storms. Despite decades of research, understanding the nuanced evolution of these clouds as they traverse the inner solar system remains a critical frontier in space weather science.

What makes this research particularly remarkable is the rare alignment of the Solar Orbiter and Wind spacecraft, positioned on the same radial trajectory toward Earth during the November 2021 CME event. This unique configuration allowed an unprecedented comparative study of the cloud’s morphology and plasma conditions at two distinct points along its trajectory, offering a window into its real-time expansion dynamics and internal heating processes. The crescent-shaped magnetic cloud, characterized by twisted magnetic flux ropes, displayed complex interactions with the ambient solar wind—a constant outflow of charged particles from the sun traveling at speeds near one million miles per hour.

The cloud’s super expansion was initially preceded by a brief compression phase upon its collision with the background solar wind. Conventional wisdom would suggest that such collisions might decelerate or compress the CME material; however, the unprecedented subsequent plasma heating drove a rapid volumetric expansion. Remarkably, while the size and temperature within the cloud changed significantly, the internal magnetic field pressure remained stable, a finding that contradicts many prevailing theoretical models. This stability in magnetic pressure suggests an intricate balance between plasma thermal dynamics and magnetic forces at play within the cloud structure.

Shirsh Soni, the study’s lead author and postdoctoral fellow specializing in solar phenomena at the University of Iowa, emphasized the rarity and importance of these measurements. The simultaneous monitoring of the magnetic cloud by both spacecraft in such a finely tuned geometric alignment is a fortuitous event seldom captured in space physics. This serendipity enabled the researchers to directly quantify the expansion velocity of approximately 192 kilometers per second—nearly double the typical expansion speeds observed in other interplanetary coronal mass ejections.

This rapid expansion velocity—equivalent to about 119 miles per second—provides insights into the energetic processes governing plasma behavior in space, highlighting the turbulent, non-linear dynamics induced by CME-solar wind interactions. This super expansion likely influences the cloud’s eventual impact on Earth’s magnetosphere and ionosphere, potentially amplifying the severity of ensuing geomagnetic storms, which can disrupt terrestrial and space-based technologies.

The study’s authors further suggest that current predictive models for space weather may need substantial revision to incorporate the possibility of such rapid and extensive expansion events. Traditional frameworks have often underestimated the extent to which plasma heating and cloud expansion can alter the morphology and impact of magnetic clouds as they approach Earth. Incorporating these findings into space weather forecasting can enhance preparedness for solar storm hazards by providing more nuanced timelines and intensity estimates for geoeffective CME arrivals.

The collaboration underlying this discovery also spans international boundaries, with key contributions from Dr. Ankush Bhaskar of the Vikram Sarabhai Space Center in India and R. Selva Kumaran from Amity University in Mumbai. Their collective expertise and data analysis brought to light the complexities of this solar event, as well as the broader implications for heliophysics. The study was partially supported by a fellowship from the University of Michigan awarded to Soni, highlighting the importance of cross-institutional collaboration in cutting-edge solar research.

Given the increasing reliance on satellite communications, GPS navigation, and power infrastructure sensitive to geomagnetic disturbances, understanding and anticipating the behavior of CMEs remains a scientific and societal imperative. This investigation’s novel approach—leveraging opportunistic spacecraft alignment and combined plasma and magnetic field measurements—sets a new paradigm for space weather observation strategies, encouraging future missions to prioritize coordinated measurements along Earth-bound solar wind streams.

Beyond immediate practical concerns, the findings enrich fundamental knowledge of plasma physics and magnetohydrodynamic processes in the heliosphere. Magnetic clouds act as natural laboratories to explore how magnetic fields and charged particles interact in extreme conditions, and observations such as these challenge researchers to rethink existing physical models and assumptions about energy transfer in space plasma environments.

This study culminates in a detailed analysis titled “Super expansion of interplanetary coronal mass ejection observed by Solar Orbiter and Wind spacecraft within 0.14 AU radial separation,” which is now accessible to the scientific community. It underscores the importance of continuous monitoring and innovative multi-point observations of solar phenomena to unravel the complex mechanisms modulating space weather and its terrestrial impacts.

As we move further into an era shaped by space technology and exploration, decoding the sun’s influence on Earth is paramount. This landmark research presents a vivid example of how meticulous observation and fortuitous spacecraft positioning can decode the dynamic and sometimes unpredictable nature of solar-driven magnetic structures, propelling our capacity to anticipate and mitigate disruptions from space weather events.

Subject of Research: Not applicable

Article Title: Super expansion of interplanetary coronal mass ejection observed by Solar Orbiter and Wind spacecraft within 0.14 AU radial separation

News Publication Date: 24-Apr-2026

Web References: http://dx.doi.org/10.1093/mnras/stag350

References:
Soni, S., Miles, D., Bhaskar, A., Kumaran, R. S. (2026). Super expansion of interplanetary coronal mass ejection observed by Solar Orbiter and Wind spacecraft within 0.14 AU radial separation, Monthly Notices of the Royal Astronomical Society.

Image Credits: David Miles lab, University of Iowa

Keywords

Solar physics, coronal mass ejection, magnetic cloud, plasma expansion, space weather, Solar Orbiter, Wind spacecraft, interplanetary medium, geomagnetic storm, heliophysics, magnetohydrodynamics, plasma heating