Astronomers have shown that even weak and nearly invisible solar eruptions can trigger intense geomagnetic storms on Earth, complicating space weather forecasting. The findings are based on a detailed investigation of a stealth Coronal Mass Ejection (CME) that originated from the Sun in March 2023 and caused a strong geomagnetic storm around three days later.

Unlike typical CMEs, which are accompanied by X-ray flares or radio bursts, this event showed no obvious warning signs on the solar disk. Such ‘stealth CMEs’ account for nearly 10 per cent of intense geomagnetic storms, making them particularly difficult to detect with existing observation systems.

The study was carried out by scientists at Indian Institute of Astrophysics, an autonomous institute under the Department of Science and Technology. According to lead author P Vemareddy, these weak eruptions leave no detectable signatures near the Sun, yet can evolve into disruptive space weather events by the time they reach Earth.

Using data from multiple spacecraft including NASA’s Solar Dynamics Observatory, Solar Orbiter, STEREO-A and WIND, the researchers traced the CME’s journey through a nearby coronal hole—an open region of the Sun’s magnetic field that allows high-speed solar wind to escape. The presence of this coronal hole is believed to have helped the CME propagate efficiently from the Sun to Earth, rather than dissipating earlier.

The interplanetary CME was observed without a clear shock front but showed expanding magnetic cloud structures, increasing radial size and enhanced plasma density. These features, combined with a southward magnetic field component, played a critical role in driving the intense geomagnetic storm.

The team also modelled geomagnetic storm intensity using solar wind velocity, density and magnetic field parameters, finding strong agreement with observed geomagnetic indices. The results highlight how subtle solar events can evolve dynamically during propagation, posing significant challenges for predicting space weather impacts on satellites, communications and power grids.

The research, co-authored by K Selva Bharathi, was published in The Astrophysical Journal, strengthening understanding of how stealth CMEs influence near-Earth space and underscoring the need for improved solar monitoring and forecasting capabilities.