Gigantic 'letter S' spotted on the sun just before a 'dark eruption' hurls a fiery shadow at Earth

A gigantic, backwards "letter S" recently appeared slap bang in the middle of the sun, moments before our home star unleashed a massive plume of plasma that later smashed into Earth, temporarily disrupting our planet's magnetic field.

The bizarre S-shaped structure, which bears a resemblance to the iconic symbol of Superman, took shape on Sept. 4, less than an hour before an M-class solar flare launched a giant magnetized cloud of plasma stretching up to 435,000 miles (700,000 kilometers) across, Spaceweather.com reported. This plasma plume, known as a coronal mass ejection (CME), appeared dark against the sun's surface because its plasma is significantly cooler than its surroundings, similar to why sunspots appear black.

This "dark eruption" smashed into Earth's magnetic field, or magnetosphere, on Sunday (Sept. 7), temporarily disturbing our planet's protective shield. This disturbance, known as a geomagnetic storm, occurred around the same time as a "blood moon" total lunar eclipse, which was visible across large parts of the globe. However, the storm only reached G1 (minor) status, meaning it did not generate many visible auroras in the U.S. or at similar latitudes, according to Live Science's sister site Space.com.

This rare type of solar outburst is known as a "sigmoid eruption," named after the Greek letter sigma, which corresponds phonetically to "S" in English. In this case, the "S" appeared just above the sun's equator, right in the center of the hemisphere facing Earth, and was roughly 78,000 miles (125,000 km) across at its widest point — around 10 times the diameter of Earth.

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Sigmoid eruptions occur when the localized magnetic fields surrounding sunspots get twisted up "like a slinky" at each end of the bar that runs through their center, causing the entire structure to snake, according to Spaceweather.com.

The shape makes it almost certain that the sunspot will create a solar flare because the magnetic field holding it together more readily snaps when the opposite ends of the magnetic field are closer together, flinging plasma away from the sun in the process, Spaceweather.com representatives added. "When you see an 'S' on the sun, it usually means something is about to explode," they wrote.

A similar structure was also seen shortly before a supercharged X9.3 magnitude solar flare in September 2017, which, at the time of the event, was the most powerful solar explosion in almost a decade, according to a 2024 study.

Researchers first noticed the explosive potential of sigmoid eruptions in the late 2000s. However, they are still trying to work out exactly what causes them to take this shape.

The classical explanation for the S shape is that it emerges when two J-shape structures — the most common shape that precedes solar flares — merge end-to-end into a single entity. However, a 2022 study hinted that it is more likely that a single J-shape structure is transformed into an S via "slipping" of its associated magnetic field.

Increased solar activity

Solar activity has been peaking over the last few years due to the solar maximum — the most active phase of the sun's roughly 11-year solar cycle, when the number and size of sunspots and solar storms rise sharply.

This explosive peak was initially predicted to be fairly tame compared to recent solar maxima. However, the initial forecasts were inaccurate, and the current maximum arrived early and has been far more active than expected.

Solar maximum has now likely come to an end. However, solar activity is expected to remain relatively high in the coming months due to continued instability within our home star's magnetic fields.

The last few weeks have seen a mini-resurgence in space weather events, including a giant "solar tornado" that raged above the sun's surface for several days in late August, and a "cannibal" CME that painted auroras above multiple U.S. states on Labor Day (Sept. 1).