Sun Whispers, Earth Feels The Storm: The Stealth Solar Eruption That Challenged Space Weather Forecasting

Bengaluru: In March 2023, a seemingly weak solar eruption travelled silently from the Sun to Earth through a coronal hole—without flares, radio bursts, or triggering early warnings. Yet within three days, it sparked an intense geomagnetic storm around our planet.

The event has since drawn significant scientific attention. In a recent study published in The Astrophysical Journal, researchers P Vemareddy of the Indian Institute of Astrophysics (IIA) and K Selva Bharathi of Indian Institute of Science Education and Research (IISER) Tirupati examined how a subtle “stealth” Coronal Mass Ejection (CME) triggered a powerful geomagnetic storm on Earth.

Coronal Mass Ejection blast and subsequent impact at Earth (Steele Hill/ NASA)

The study not only increased the challenges of long-held assumptions about how space weather events originate but also raised concerns about the limitations of current forecasting systems.

What is a CME and why does it matter?

CMEs are massive expulsions of plasma and magnetic fields from the Sun’s atmosphere. When directed toward Earth, they can interact with the planet’s magnetosphere and generate geomagnetic storms capable of disrupting satellites, GPS systems, aviation routes, communication networks, and power grids.

However, about 10 per cent of intense geomagnetic storms do not arise from large, clearly visible eruptions. Instead, they are caused by weak or “stealth” CMEs, events so subtle that they are often missed in routine observations. Understanding these stealth CMEs is crucial for improving space weather prediction.

The 19 March 2023 stealth CME

The CME investigated in this study occurred on 19 March 2023. It emerged from the eruption of a longitudinal filament channel near the Sun’s centre. Unlike typical strong CMEs, it was not accompanied by X-ray flares or radio bursts, making it exceptionally elusive.

Although it appeared large in size, it was weak in intensity compared to most other eruptions and was not expected to reach Earth. Yet nearly three days later, Earth experienced an intense geomagnetic storm. To investigate, researchers analysed data from multiple spacecraft, including NASA’s Solar Dynamics Observatory (SDO), Solar Orbiter (SolO), STEREO-A, and WIND. The study examined both remote observations near the Sun and in situ measurements in interplanetary space.

Explaining the challenge of identifying the event in an exclusive interview with ETV Bharat, P Vemareddy said, “Such weak CMEs leave no detectable signatures on the Sun and hence are extremely difficult to identify with current observational sensitivity.”

He further explained, “Initially, we identified a weak CME in the observational data and confirmed its presence. At the same time, there was another CME that appeared to be a possible candidate for causing the storm. However, detailed analysis showed that this second CME was travelling in a different direction within the heliosphere and was not Earth-directed. After ruling it out as the source, we relied on further observational data from multiple instruments to confirm that the weak CME was indeed responsible for the storm.”

Forecasting failure

On advance alerts, Vemareddy stated, “We did not have an early warning in this case. Advance alerts are usually triggered by two key signatures associated with strong CMEs: large solar flares and Type II radio bursts. These signals can provide about 8–10 minutes of warning before the disturbance reaches Earth, but this CME produced neither."

“However, this was a weak CME, and it did not produce either a significant flare or a Type II radio burst. Since those typical warning signatures were absent, no advanced warning could be issued in this case,” he noted.

Discussing the forecasting challenges, Vemareddy explained that the main difficulty lay in the fact that geomagnetic storms were usually predicted when a strong CME was clearly observed erupting from the Sun. In such situations, scientists could estimate its speed, direction, and likely impact on Earth using observational images. However, he noted that this case was different. The CME involved was weak, and the disturbance was barely visible in solar images—even in detailed observations. Because it was not clearly detected, it was initially overlooked, and the predictions failed. He added that earlier forecasts had misidentified another CME as the possible source, but that one later turned out to be travelling in a different direction.

He added, “Such weak and poorly observed CMEs create major difficulties for forecasting. Whether predictions are model-based or AI-driven, these missed events become outliers and can lead to incorrect forecasts. When observational limitations exist, weak CMEs can unexpectedly hit Earth, causing prediction models to fail.”

Coronal Hole: A hidden accelerator

First, a coronal hole existed near the CME’s source region on the Sun. Since the Sun is an extended object, scientists can pinpoint the exact location from where a CME is launched. The nearby coronal hole—a region with open magnetic field lines from which high-speed solar wind streams escape—played a crucial role. The interaction between the weak CME and the solar wind from this coronal hole helped propel and guide the eruption toward Earth.

“Because this weak CME erupted near a coronal hole, it had a clear path outward along these open magnetic field lines. The fast solar wind flowing from the coronal hole, typically around 600–700 km/s, helped push and accelerate the CME. Although the CME was initially moving at about 300 km/s, it gained speed, reaching nearly 550 km/s, as it travelled along with the fast solar wind. This acceleration further confirmed how it was able to propagate all the way to Earth.”

Using spacecraft observations, scientists tracked the CME at about 0.5 AU (Astronomical Unit) or halfway between the Sun and the Earth. In this case, interaction with the high-speed solar wind helped it survive the journey to 1 AU—the distance between the Sun and the Earth.

By observing it at this midway point and then again near Earth, researchers confirmed that the same weak CME had travelled the entire distance. The interplanetary CME (ICME) was travelling behind a high-speed solar wind stream and was detected without a clear shock or sheath, features normally associated with strong events.

Are We Underestimating Small Solar Eruptions?

The research team analysed data spanning two to three solar cycles—roughly 25 years—and identified at least six similar overlooked events. Vemareddy acknowledged, “Generally, weak CMEs are ignored because most of them dissipate within 30–40 solar radii, or by about 0.1–0.2 AU, and never reach Earth. From a scientific perspective, such small-scale disturbances are often studied, but they are not considered a major threat to Earth’s magnetosphere or space weather. However, in our analysis of data spanning the last 2–3 solar cycles, we identified at least six similar events in past records.

"These were largely overlooked. If we slightly lower the detection threshold, such weak events could account for nearly 10 per cent of total CME-related impacts," he added.

Another critical aspect was the CME’s southward magnetic orientation. Vemareddy explained, “Earth’s magnetic field is directed northward with respect to the ecliptic plane—the plane in which the Sun and Earth orbit. If the magnetic field carried by a CME is directed southward, it becomes anti-parallel to Earth’s northward field. This opposite alignment allows magnetic reconnection to occur, enabling efficient transfer of energy from the solar wind into Earth’s magnetosphere. Such southward-oriented CMEs therefore have a much higher potential to trigger strong geomagnetic storms.”

The March 2023 CME carried such a southward component, increasing its geoeffectiveness. Refuting possibiolity of stealth CMEs becoming more frequent, Vemareddy said, “The growing attention is largely due to increased scientific focus and improved data analysis. More researchers are now examining existing datasets from different perspectives, which has brought these previously overlooked events into sharper focus.”

Responding to whether artificial intelligence can help with detection, he said, “It is currently very difficult. AI and ML models require large volumes of historical data to be properly trained.”

A depiction of the Sun’s magnetic fields overlaid on an image of the Sun captured in extreme ultraviolet light by NASA’s Solar Dynamics Observatory (File Photo: NASA/SDO/AIA/LMSAL)

He continued, “However, stealth CMEs leave very weak or unclear signatures in observational data. Since past datasets do not consistently capture these subtle features, there is insufficient training material for AI systems to reliably detect them. Without clear patterns in the data, even advanced algorithms may struggle to identify such events in advance.”

A new understanding of the Sun–Earth connection and the way forward

Talking about the broader implications, Vemareddy said that the study challenges the long-held view that geomagnetic storms are mainly caused by large CMEs with clear observational signatures.

He further said, “This study shows that even weak or ‘stealth’ CMEs, which are difficult to detect, can travel to Earth and trigger storms. It suggests that we may be missing important Sun–Earth connections due to observational limitations. The role of open magnetic field regions, such as coronal holes, can provide a pathway that helps these weak disturbances propagate efficiently toward Earth.”

To improve detection, he emphasised, “There is a need for more advanced observational studies using new satellite missions. He suggested deploying satellites at additional vantage points such as L4 and L5, and possibly placing another spacecraft closer to the Sun, potentially by India. Such missions would improve heliospheric observations and help the scientific community better understand and track weak solar disturbances. Greater focus and coordinated efforts are needed to explore these new avenues in space weather research.”