A solar flare can send a shock wave racing across the Sun’s outer atmosphere even when the star does not launch a coronal mass ejection, according to a study that challenges the assumption that the most dramatic extreme-ultraviolet waves must be driven by giant clouds of plasma escaping into space. By comparing large-scale coronal propagating fronts with and without accompanying coronal mass ejections, researchers have identified distinct patterns in wave speed, flare impulsiveness, energy release and plasma emission. The findings suggest that confined flares—events in which magnetic energy is released but the overlying magnetic structure remains locked to the Sun—can generate genuine coronal shock waves through a mechanism different from the one associated with eruptive flares.

The result matters because large-scale waves in the corona are more than spectacular arcs sweeping across solar images. They can disturb magnetic fields over enormous distances, compress and heat plasma, and potentially alter the paths of energetic particles. Astronomers have long debated what launches these fronts, which are often called extreme-ultraviolet, or EUV, waves. In many events, the wave appears alongside a coronal mass ejection, making the expanding CME a natural suspect: as billions of tonnes of magnetized plasma surge outward, they can drive a shock through the corona much as a supersonic aircraft generates a bow shock in the atmosphere. But some waves appear without any detectable CME. Their existence raises a fundamental question: are these “standalone” fronts merely slower disturbances caused by the flare, or can a flare itself create a shock wave?

Robert Bush of Tufts University, John Stefan of the New Jersey Institute of Technology and Alexander Kosovichev of NJIT and NASA Ames Research Center investigated that question by comparing different populations of solar eruptions. The team used extreme-ultraviolet observations from the Atmospheric Imaging Assembly aboard NASA’s Solar Dynamics Observatory to identify large-scale coronal propagating fronts, or LCPFs. These observations capture the Sun in several EUV wavelengths, allowing scientists to follow changes in the hot, tenuous plasma suspended above the visible surface. To determine whether a wave was associated with an eruption, the researchers examined coronagraph images from the Large Angle and Spectrometric Coronagraph, or LASCO, aboard the Solar and Heliospheric Observatory. A coronagraph blocks the bright solar disk, making the much fainter material of a CME visible as it expands into the surrounding heliosphere.

The comparison revealed a clear kinematic divide. LCPFs linked to CMEs propagated noticeably faster than those observed without a CME. In physical terms, the CME-associated fronts are consistent with a disturbance driven by the rapid expansion of a large magnetic-plasma structure. The expanding ejecta pushes against the ambient corona, where the local fast-mode magnetosonic speed—the characteristic speed of a combined magnetic and acoustic disturbance—depends on both the plasma temperature and the strength of the magnetic field. When the driver moves faster than the surrounding medium can respond, it can steepen into a shock. Such shocks compress plasma and modify the magnetic field as they travel, producing the moving EUV brightening seen by spacecraft. The slower standalone waves, however, cannot simply be treated as weaker versions of CME-driven shocks; their different speeds point to a separate source of momentum and energy.

To examine that source, the researchers focused on “standalone flare events,” also known as confined flares. These events produced coronal waves but showed no corresponding CME in LASCO observations. The team analyzed soft X-ray measurements from NOAA’s Geostationary Operational Environmental Satellites, or GOES, which monitor the Sun’s X-ray output continuously. Soft X-rays provide a useful record of flare evolution: the rise of the emission traces heating and plasma filling, while the time derivative of the flux acts as a proxy for the rate at which energy is being released. The scientists compared the impulsive phase duration, the characteristic energy-release time, the maximum derivative of the soft X-ray flux, the peak flux, the flare energy, the plasma temperature and the volume emission measure across flare populations.

Volume emission measure is particularly informative because it estimates how much hot plasma is present along the line of sight. Technically, it is related to the integral of the square of the electron density over the emitting volume, commonly expressed in units of inverse cubic centimetres. A dense, hot flare loop can therefore produce a large emission measure even if its total geometric volume is difficult to determine. The GOES analysis also requires assumptions about elemental abundances—whether the plasma has photospheric composition resembling the solar surface or coronal composition altered by processes in the upper atmosphere. The team calculated temperatures and emission measures under both abundance assumptions, allowing them to test whether the differences between flare classes were robust rather than artifacts of the diagnostic model.

The confined flares turned out to occupy an intermediate physical regime. Compared with flares known to generate sunquakes, they were less impulsive and less energetic. Sunquakes are seismic ripples launched into the solar interior, detected through subtle changes in the Doppler motions of the photosphere. A leading explanation proposes that flare-accelerated particles, especially energetic electrons or protons, stream downward and deposit energy in the dense lower atmosphere. The resulting sudden heating and pressure increase can push into the photosphere, exciting acoustic waves below the surface. Numerical models of this particle-driven process predict that energy deposition might also launch a wavefront through the corona and a Moreton–Ramsey wave through the chromosphere, the atmospheric layer immediately above the photosphere.

The new comparison complicates that unified picture. Standalone flare events were more impulsive than LCPF-associated flares that also produced CMEs, but they were less energetic overall. Most significantly, CME-associated coronal waves exhibited substantially higher volume emission measures. That excess hot plasma is consistent with a large eruptive structure contributing directly to the observed disturbance. Confined flares, by contrast, release energy rapidly enough to perturb the corona but do not produce the massive, high-emission-measure response characteristic of an expanding CME. The wave in these cases may arise from the flare’s sudden energy release, magnetic restructuring or pressure changes in the lower atmosphere, rather than from a large body of plasma plowing outward through the corona.

The authors caution that the observations do not prove a single microscopic mechanism for every standalone wave, nor do they eliminate the possibility that some weak or poorly oriented CMEs escaped detection. Coronagraph observations have limited sensitivity and geometry can hide an eruption directed toward or away from the observer. Even so, the population-level differences are difficult to explain if all EUV fronts share the same origin. The study provides a practical way to separate wave-producing events: speed, flare impulsiveness and emission measure together reveal whether a front is more likely to be CME-driven or generated by a confined flare. That distinction could improve forecasts of how solar disturbances spread across the corona and help researchers interpret the relationship between flares, coronal shocks and sunquakes.

The work also offers a new perspective on the Sun’s ability to produce powerful effects without fully erupting. Solar activity is often imagined as a binary choice between a contained flare and a CME blasting into interplanetary space. In reality, the study indicates that magnetic energy can be converted into large-scale waves even when the magnetic configuration prevents the bulk atmosphere from escaping. Future observations that combine EUV imaging, magnetic-field measurements, hard X-rays, radio emissions and helioseismic data may determine whether energetic particles, chromospheric pressure pulses or rapidly reorganizing coronal fields provide the decisive push. For now, the message from the Sun is striking: an explosion does not need to leave the star to send a shock wave across it.

For decades, researchers have known that the Sun is an immense natural particle accelerator. It accelerates electrons to velocities approaching the speed of light, launching them outward in immense numbers that flood the entire solar system. These highly energized particles, termed Solar Energetic Electrons (SEEs), have long been observed, but their precise origins and the mechanics of their acceleration remained elusive—until now.

Utilizing the unprecedented capabilities of Solar Orbiter, a collaborative mission between ESA and NASA, scientists have observed over 300 separate SEE bursts between November 2020 and December 2022. This extensive dataset, gathered closer to the Sun than any previous mission could venture, has allowed researchers to map these electrons back to two distinct solar phenomena: impulsive solar flares and gradual coronal mass ejections (CMEs). Each source imprints unique signatures on the energetic electrons it emits.

Solar flares, explosive events localized to relatively small regions on the Sun’s surface, generate rapid, intense bursts of energetic electrons. These impulsive events are akin to sudden flares of light and particle radiation shooting out in quick succession. In contrast, CMEs represent massive eruptions where immense clouds of hot plasma and magnetic fields are expelled from the Sun’s atmosphere. Electrons associated with CMEs manifest more gradual releases, spreading over extended timeframes and producing a broader ‘swell’ of energetic particles traveling through the solar system.

A defining feature of this research is the ability to trace electron particles from their earliest detectable stages near the solar surface, thanks to Solar Orbiter’s suite of sophisticated instruments. This proximity provides a ‘pristine’ measurement environment, enabling precise determinations of where and when these electrons originate. No previous mission had the instrumentation or vantage point to confidently delineate these nuanced differences between impulsive and gradual SEE populations.

Another longstanding puzzle tackled by Solar Orbiter’s data concerns the apparent time delays observed between solar events and the subsequent detection of energetic electrons in space. These delays sometimes span hours, raising critical questions about the particle acceleration and propagation processes in the heliosphere. The new findings highlight that such lag arises not solely from delayed electron release at the Sun but also from the complex journey electrons undertake through the turbulent magnetic environment of the solar wind.

The solar wind—a continuous outflow of charged particles emanating from the Sun—carries the Sun’s magnetic field outward, filling the vast expanse of interplanetary space. This dynamic medium neither allows electrons to travel unimpeded nor along simple trajectories. Instead, electrons are continually scattered, deflected, and trapped by magnetic turbulence and irregularities, causing staggered arrival times and complicating detection.

These complexities underscore the significance of Solar Orbiter’s multi-instrument, multi-distance observational strategy. By measuring SEE events at varying points between the Sun and Earth, the mission effectively disentangles source characteristics from transport effects. This approach provides a more holistic understanding of how energetic particles evolve from their solar origins to their manifestations in near-Earth space and beyond.

The implications of this research are profound for space weather forecasting. Among the two categories of SEE, those associated with CMEs are particularly impactful due to their higher energy content and more extended particle emissions. CME-related particle swells have a far greater potential to damage satellites, disrupt communications, and pose radiation hazards to astronauts. Accurately distinguishing between electron populations therefore enhances predictive capabilities, enabling earlier alerts and improved risk management for space operations.

Mission scientists emphasize the collaborative nature of this breakthrough, combining expertise and instrument data from diverse European and US teams. The comprehensive catalog of SEE events, named CoSEE-Cat, has been made publicly accessible to the global scientific community. This repository offers invaluable opportunities for further multi-disciplinary analyses and will likely serve as a benchmark dataset for years to come.

Looking to the future, ESA’s planned Vigil mission promises to complement Solar Orbiter’s achievements by providing continuous, side-on observations of the Sun starting in 2031. Vigil aims to detect dangerous solar eruptions well before they rotate into Earth’s line of sight, enabling even earlier warning systems. Meanwhile, ESA’s Smile mission, launching soon, will probe how solar wind particles interact with Earth’s magnetic shield, furthering our grasp of planetary space weather dynamics.

Together, these missions form an integrated approach to constantly monitor the Sun-Earth environment, shedding light on the mechanisms driving solar storms and energetic particle events. As our reliance on space infrastructure deepens, understanding and forecasting these solar phenomena become ever more critical to safeguarding technology, communication networks, and human space exploration endeavors.

Solar Orbiter’s revelations about energetic electron sources mark a significant leap in heliophysics, illustrating the tremendous value of close-up solar observations. By disentangling the intricate threads connecting solar eruptions to particle acceleration and propagation, scientists are now better equipped to decode the Sun’s complex influence on the solar system, an influence that ultimately shapes conditions here on Earth.

Subject of Research: Not applicable
Article Title: CoSEE-Cat: a Comprehensive Solar Energetic Electron event Catalogue obtained from combined in-situ and remote-sensing observations from Solar Orbiter
News Publication Date: 1-Sep-2025
Web References: http://dx.doi.org/10.1051/0004-6361/202554830
References: Warmuth, A. et al. “CoSEE-Cat: a Comprehensive Solar Energetic Electron event Catalogue obtained from combined in-situ and remote-sensing observations from Solar Orbiter.” Astronomy & Astrophysics, 2025.
Image Credits: ESA & NASA/Solar Orbiter/STIX & EPD

Keywords

Solar Orbiter, Solar Energetic Electrons, Solar Flares, Coronal Mass Ejections, Space Weather, Particle Acceleration, Heliosphere, Solar Wind, Energetic Particles, Solar Physics, ESA, NASA