New analysis suggests rare limb-origin solar eruptions can trigger severe geomagnetic storms

A study published on August 23, 2025, in Space Weather re-examines the long-held assumption that solar eruptions close to the limb cannot generate severe geomagnetic disturbances on Earth. The research focuses on two major historic events: the Fátima storms of January 1938 and the Halloween storms of October–November 2003.

Coronal Mass Ejections (CMEs) launched from the limb are generally thought to miss Earth, with most studies showing they produce at most modest geomagnetic effects. However, the second Fátima storm produced one of the most intense geomagnetic and auroral disturbances in modern records, despite originating from a sunspot group very close to the solar limb.

The event caused geomagnetic indices to reach disturbance levels that have occurred less than 0.01% of the time since 1932, with auroras observed as far south as Tataouine, Tunisia. This makes it the 13th most extensive auroral event recorded since the Maunder Minimum (circa 1645–1715).

In contrast, the Halloween storms of 2003 included what is estimated to be the most powerful solar flare on record, but because it erupted just beyond the eastern limb, the associated CME missed Earth and caused no significant geomagnetic disturbance.

The study proposes two possible explanations for the 1938 storm. The first is that the CME launched from the limb had an unusually large longitudinal extent or was deflected toward Earth. The second is that another, less visible CME closer to the solar disk, possibly a so-called “stealth CME,” was responsible. Due to limited data from the 1930s, the exact cause cannot be determined with certainty.

Researchers also examined substorm activity during the Fátima event, noting that the unusually widespread red auroras at low latitudes likely resulted from a major enhancement of the ring current. This aligns with modern understanding of how geomagnetic storms drive substorm activity and low-latitude auroral displays.

The authors conclude that while limb-origin eruptions very rarely generate major geomagnetic storms, the 1938 case shows that they cannot be fully dismissed in space weather forecasting.

References:

¹ Can a Major Geomagnetic and Auroral Disturbance Originate From a Solar Active Region Close to the Limb? – Mike Lockwood et al. – Space Weather – August 2025 – https://doi.org/10.1029/2025SW004517 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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