Stealth Solar Blast Triggers Major Geomagnetic Storm, Study Warns

Indian astronomers uncover how faint solar eruptions can cause outsized space-weather impacts on Earth.

Astronomers have uncovered compelling new evidence that even the Sun’s faintest eruptions can unleash powerful geomagnetic storms on Earth—posing fresh challenges for global space-weather forecasting.

In a study published in The Astrophysical Journal, researchers reveal how a nearly invisible “stealth” Coronal Mass Ejection (CME) on 19 March 2023 travelled from the Sun to Earth and triggered an intense geomagnetic storm just three days later. Crucially, the event occurred without the usual solar warning signs such as X-ray flares or radio bursts, meaning it could have easily gone unnoticed.

Coronal Mass Ejections are massive expulsions of solar plasma and magnetic fields that can disrupt satellites, navigation systems, radio communications, and power grids. While most severe geomagnetic storms are linked to large, easily observable CMEs, around 10% arise from weak or stealthy eruptions that current monitoring systems often fail to detect.

The new study shows that these subtle events can be just as dangerous.

A hidden eruption with global consequences

The stealth CME originated from a filament channel near the centre of the Sun’s disk and travelled through a nearby coronal hole—an opening in the Sun’s magnetic field that allows high-speed solar wind to escape. This coronal hole acted like a cosmic conveyor belt, helping the weak CME maintain its structure and momentum all the way to Earth.

“Such weak CMEs leave no detectable signatures on the Sun and hence are extremely difficult to identify with current observational sensitivity,” said Dr P. Vemareddy, lead author of the study and scientist at the Indian Institute of Astrophysics (IIA), an autonomous institute under the Department of Science and Technology, Government of India.

Using coordinated observations from multiple spacecraft—NASA’s Solar Dynamics Observatory (SDO), Solar Orbiter (SolO), STEREO-A, and WIND—the team tracked the CME’s evolution across the heliosphere. Despite lacking a clear shock or sheath, the interplanetary CME (ICME) carried a southward magnetic field component and enhanced plasma density, key ingredients for driving strong geomagnetic storms on Earth.

Why forecasting just got harder

In situ measurements showed that the magnetic cloud expanded as it moved outward, with its size increasing from 0.08 AU near Solar Orbiter to 0.18 AU near Earth. The magnetic field also rotated during propagation, maintaining a right-handed helicity consistent with its solar source.

By modelling geomagnetic indices using solar wind velocity, density, magnetic field, and electric field data, the researchers found strong agreement between simulated and observed storm intensity—especially when density and electric field variations were included.

The findings highlight a critical blind spot in current space-weather prediction systems: solar eruptions that appear insignificant near the Sun can evolve into major threats by the time they reach Earth.

“This study underscores the urgent need to improve detection and modelling of stealth CMEs,” the authors note, warning that reliance on visible solar eruptions alone may leave Earth vulnerable to unexpected space-weather shocks.

A call to action for space-weather stakeholders

With societies increasingly dependent on satellite infrastructure, GPS-based services, and resilient power grids, the implications are far-reaching. The researchers call on space-weather agencies, satellite operators, and early adopters of forecasting technologies to integrate multi-spacecraft observations and density-aware models into operational systems.

Better preparedness, they argue, will depend on recognising that the Sun’s quietest eruptions may sometimes be its most deceptive.

The study, titled “An Intense Geomagnetic Storm Originated from Stealth Coronal Mass Ejection: Remote and In Situ Observations by Near Radially Aligned Spacecraft,” is authored by P. Vemareddy (IIA) and K. Selva Bharathi (IISER Tirupati), an MSc intern at IIA.