On September 1, 1859, British astronomer Richard Carrington was drawing a significant cluster of sunspots when he noticed two areas of bright light emerging within them. He observed them intensifying, diminishing, and vanishing within approximately five minutes.
About 17 hours later, the Earth’s magnetic field erupted. Auroras expanded into tropical regions. Telegraph systems malfunctioned throughout Europe and North America. Sparks erupted from equipment, paper reportedly ignited, and some operators experienced electric shocks.
This occurrence, now known as the Carrington Event, sets the standard for a severe geomagnetic storm. A reoccurrence would confront a world increasingly reliant on electricity and signals from outer space, but it wouldn’t function as an electromagnetic switch that instantly disables every phone, vehicle, and computer.
Carrington observed the Sun glowing in visible light
Carrington was examining a projected image of the Sun, a technique that enabled him to chart sunspots without directly looking through the telescope. Another English observer, Richard Hodgson, independently witnessed the same flash.
Carrington’s initial report in the Monthly Notices of the Royal Astronomical Society detailed two regions of “intensely bright and white light.” It represented the first intricate observation of what scientists currently refer to as a white-light solar flare.
A flare signifies an explosion of electromagnetic radiation. The ensuing geomagnetic storm was triggered by magnetized material released from the Sun, now identified as a coronal mass ejection, or CME. Light from a flare reaches Earth in approximately eight minutes. A CME is material and travels significantly slower, though the arrival in 1859 seems to have been exceptionally prompt.
The telegraph network inadvertently became a detector
Telegraph lines were among the longest electrical conductors at that time. As the Earth’s magnetic field fluctuated rapidly, it generated electric fields in the ground. These fields induced unwanted currents within grounded wires.
The effects varied among lines and locations. Some systems ceased functioning. Others emitted sparks, shocked operators, or became so highly energized that their regular batteries disrupted communications.
A remarkable collection of observations was compiled shortly after in the American Journal of Science. Operators on a line between Boston and Portland disconnected their batteries and continued transmitting messages using the current generated by the aurora. The current fluctuated on its own, at times becoming too intense for their relay magnets.
Consequently, the well-known shocks are not a later fabrication. They align with contemporary accounts and with the same induction physics relevant for power-grid engineers today.
Auroras reached as far as Cuba and Hawaii
Auroras typically congregate around high geomagnetic latitudes, where charged particles directed by the Earth’s magnetic field collide with gases in the upper atmosphere. An intense storm expands the auroral regions toward the equator.
During the 1859 event, observers reported auroras from areas that seldom experience them. The US National Weather Service records instances as far south as Cuba and Hawaii. Newspapers described red skies and luminescence bright enough in some places to read by.
The geographical extent holds more scientific significance than the hue of any individual report. Low-latitude auroras demonstrate how profoundly the storm disturbed the magnetosphere, although reconstructing its precise strength from 19th-century instruments and eyewitness testimonies contains considerable uncertainty.
A modern grid faces its own version of the telegraph challenge
A geomagnetic storm doesn’t need to strike a transformer like lightning. It alters the magnetic field over a vast area. The resulting geoelectric field can induce current through lengthy transmission lines and into grounded transformer windings.
These geomagnetically induced currents can disrupt transformer functioning, generate excessive heating, confuse protective devices, and complicate voltage management. Failures in one section of an interconnected network can impose additional stress on another.
On NOAA’s G1-to-G5 geomagnetic storm scale, a G5 event can generate extensive voltage-control and protection issues. Certain grid systems may experience blackouts, and transformers may sustain damage. “May” is crucial: the result hinges on storm duration and orientation, grid design, operational choices, and the electrical conductivity of the substrate beneath each region.
A 2025 US Geological Survey study simulated a Carrington-class storm across the United States. It identified particularly strong potential geoelectric fields in parts of the East and Midwest, while values could vary sharply even within a single state due to underground geology’s influence on current flow.
Satellites, navigation, and radio would encounter various hazards
Spacecraft do not encounter the ground currents that threaten lengthy power lines. They contend with charged particles, surface charging, radiation effects, and an upper atmosphere heated and expanded by the storm.
NOAA indicates that an extreme geomagnetic storm can lead to issues with satellite positioning, tracking, uplinks, and downlinks. Increased atmospheric drag could change the paths of low-Earth satellites. Operators might need to adjust spacecraft modes or rectify those orbits afterward.
The disturbed ionosphere can also impair satellite navigation for several days and render high-frequency radio communication unfeasible in numerous areas for one or two days. Aviation, maritime operations, surveying, and precision agriculture can all rely on those services, though