A deep red band of light stretched across the night sky on October 18, seen from Maine to Sweden and Finland. The phenomenon followed a moderate G2-class geomagnetic storm caused by a coronal mass ejection grazing Earth’s magnetosphere. While the storm’s strength was typical, the optical display was not.

“My son and I were surprised by this red bow in the south,” said P-M Hedén, who photographed the emission from Norrtälje, Sweden. “It was so bright, I could see its reflection in a local pond.” Observers in southern Öland, Sweden; Searsport, Maine; and southern Finland captured similar images, confirming that the arc extended across the Atlantic sector.

“Only a few times in a solar cycle do we get an SAR arc this bright,” said Jeff Baumgardner of Boston University’s Center for Space Physics, who has studied the phenomenon for decades. “It nearly saturated our detectors.” He estimated the arc was 10 to 30 times brighter than a G2 storm would normally produce, a figure still awaiting calibrated data.

SAR arcs, or Stable Auroral Red arcs, occur when thermal energy from Earth’s ring current leaks into the upper atmosphere. The ring current is a torus-shaped flow of charged particles encircling Earth between about three and eight Earth radii, carrying electrical currents of millions of amperes.

During geomagnetic storms, part of this energy escapes along magnetic field lines and heats dense plasma near the boundary of the plasmasphere. Excited atomic oxygen then emits red light at a wavelength of 630 nanometers, producing the characteristic glow.

SAR arcs were first identified in 1956, at the dawn of the Space Age. Early researchers mistook them for auroras and named them “Stable Auroral Red arcs,” a misnomer since they do not originate from charged particles streaming in from space but from heat conduction within the ring-current region.

The red emission is difficult for human eyes to see directly because night vision is relatively insensitive to red light. Nonetheless, the October 18 arc was bright enough to be visible to the naked eye at mid-latitudes, an uncommon occurrence for a storm of moderate intensity.

According to NOAA’s space-weather scale, a G2 storm represents a moderate disturbance, typically too weak to produce such bright emissions. The brightness and geographic reach, from North America across the Atlantic to northern Europe, suggest an unusually strong energy transfer between the ring current and the ionosphere.

Comparable large-scale SAR arcs have been documented only a few times in recent decades, notably during the October 29, 1991, geomagnetic storm and a global SAR event in November 2023. The 2025 occurrence joins this short list, distinguished by its brightness despite moderate storm strength.

Researchers at Boston University and collaborating institutions are compiling ground-based and satellite observations, including data from NOAA’s GOES spacecraft and ESA’s Swarm constellation, to determine why this event was so luminous.

For now, the October 18 SAR arc demonstrates that even moderate solar disturbances can produce complex and visually striking energy exchanges within Earth’s magnetic environment. The mechanism behind this unusually bright leak remains under investigation.

References:

1 Earth’s electrical rings just sprang a leak – SpaceWeather – October 22, 2025