The longest continuous observation of a solar active region reveals how prolonged magnetic evolution drives extreme space weather

In May 2024, Earth experienced the strongest geomagnetic storm since 2003, producing widespread auroras across Europe and disrupting satellite-based technologies. The storm was traced to a single source on the Sun, an exceptionally complex active region known as NOAA 13664. For the first time, scientists were able to follow such a region almost continuously from its birth to its decay, providing new insight into how extreme space weather develops.

An international team led by researchers at ETH Zurich tracked NOAA 13664 for 94 consecutive days between April and July 2024. By combining observations from two spacecraft positioned at different vantage points around the Sun, the team reconstructed the full life cycle of one of the most eruptive solar regions ever observed.

The Sun rotates around its axis approximately once every 28 days. From Earth, this limits direct observation of any active region to about 14 days before it rotates out of view. Critical phases in a region’s evolution often occur while it is hidden on the far side of the Sun, leaving major gaps in the observational record. These gaps have long constrained scientists’ ability to understand how prolonged magnetic evolution leads to extreme solar storms.

That limitation was overcome by combining data from the European Space Agency’s Solar Orbiter and NASA’s Solar Dynamics Observatory. Solar Orbiter follows a highly elliptical orbit that periodically places it far from the Sun-Earth line, allowing it to observe regions invisible from Earth. The Solar Dynamics Observatory remains near the Sun-Earth line and continuously monitors the Earth-facing hemisphere.

Between April and July 2024, the geometry of the two spacecraft allowed near continuous coverage of NOAA 13664 for 94 days. This is the longest uninterrupted observational dataset ever assembled for a single solar active region.

NOAA 13664 first emerged on April 16, 2024, on the far side of the Sun. Early Solar Orbiter observations showed a small bipolar magnetic structure embedded in an already active environment. Over the next 20 days, repeated episodes of magnetic flux emergence occurred as strongly magnetised plasma rose from beneath the solar surface. These episodes did not occur in isolation. Multiple magnetic systems developed close together, interacting and gradually merging into a single, increasingly complex region.

By late April, the region had evolved into one of the most magnetically complex structures of Solar Cycle 25. When it rotated into Earth’s view in early May, it was already capable of producing major eruptions. Its magnetic field was strongly non-potential, meaning it deviated significantly from a relaxed, current-free configuration. Such fields store large amounts of free magnetic energy.

Measurements showed extremely long and densely packed magnetic polarity inversion lines, where opposite magnetic polarities meet. These structures are known to be the primary sites of powerful solar flares and fast coronal mass ejections. What made NOAA 13664 unusual was that this extreme magnetic complexity was sustained over weeks rather than days.

Between May 8 and May 12, 2024, the region produced multiple X-class flares and associated coronal mass ejections that struck Earth. The resulting geomagnetic storm was the strongest since 2003. Auroras were observed as far south as Switzerland, while satellite navigation, communication systems, and sensor-dependent technologies experienced widespread disruption.

The region’s most powerful eruption occurred on May 20, 2024, when it was once again on the far side of the Sun. An X16.5 flare, one of the strongest solar flares recorded in twenty years, was detected by Solar Orbiter instruments. In total, 969 flares were recorded from NOAA 13664 over 94 days, including 38 X- and 146 M-class events.

Solar storms inject enormous amounts of energy and charged particles into near-Earth space. These disturbances compress Earth’s magnetosphere and can induce electric currents in the upper atmosphere and along the ground. During the May 2024 storm, satellite positioning signals were disrupted, affecting aviation, agriculture, and precision timing systems. Digital farming operations relying on satellite guidance, drones, and sensors reported operational delays and economic losses.

High-frequency radio communication experienced interference, while power grids and railway signalling systems faced increased risk from geomagnetically induced currents. Similar processes led to the loss of 38 newly launched Starlink satellites in February 2022, highlighting the vulnerability of modern technology to extreme solar activity.

After reaching peak activity, NOAA 13664 entered a gradual decay phase. As it completed further solar rotations, the region fragmented into smaller magnetic structures, and flaring activity slowly declined. However, magnetic interactions and sporadic flux emergence continued into June 2024. Only by mid July did the region fully disperse into weak, scattered magnetic elements.

The near-continuous dataset shows that the most dangerous space weather events are not caused by sudden, isolated eruptions. Instead, they are the result of long-term magnetic evolution that builds over weeks to months. Regions that maintain high magnetic non-potentiality across multiple rotations pose a sustained threat, even when they are temporarily hidden from Earth.

This finding has direct implications for space weather forecasting. Continuous multi-vantage observations make it possible to identify hazardous regions earlier, track their evolution in real time, and better assess the likelihood of extreme eruptions. The strong correlation observed between magnetic complexity and flare activity confirms that long-lived magnetic stress is a key predictor of severe space weather.

The results also show the importance of future missions designed to monitor the Sun from multiple angles. The European Space Agency’s planned Vigil mission, scheduled for launch in 2031, will observe solar activity from a side-on perspective dedicated specifically to space weather monitoring. With additional spacecraft distributed around the Sun, solar magnetic surveillance could become effectively continuous, reducing blind spots and extending warning times for Earth-directed storms.

The 94-day observation of NOAA 13664 represents a milestone in solar physics. For the first time, scientists have followed a super active region from emergence to decay without losing sight of its most critical phases. The study shows that extreme space weather is rooted in sustained magnetic complexity and that understanding this long-term evolution is essential for protecting modern technological society.

References:

¹ Near-continuous tracking of solar active region NOAA 13664 over three solar rotations – I. Kontogiannis et al. – Astronomy & Astrophysics – October 3, 2025 – https://doi.org/10.1051/0004-6361/202556136 – OPEN ACCESS

² Longest observation of an active solar region – ETH Zurich – January 5, 2025

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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