Solar Flares Can Launch Coronal Shock Waves Without Exploding Into Space

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.