A Carrington-level event is a geomagnetic storm comparable in intensity to the one triggered by a massive solar eruption in September 1859, when an explosion on the Sun plunged telegraph systems into chaos and lit up the sky with aurora visible across roughly two-thirds of the Earth’s surface.1PubMed. Astronomical fire: Richard Carrington and the solar flare of 1859 In modern terms, it has become shorthand for a worst-case space weather scenario, the kind of storm that could knock out power grids, disable satellites, and scramble GPS signals for hours or days. The probability that one will strike within any given decade is low but not negligible, and the consequences for a civilization now wired with hundreds of thousands of kilometers of conductors and reliant on orbiting electronics would be vastly more severe than anything the telegraph age experienced.
What Happened in 1859
On the morning of September 1, 1859, English amateur astronomer Richard Carrington was sketching sunspots when he witnessed an intensely bright flash erupt from the Sun’s surface. It was the first recorded observation of a solar flare. From his original drawings, researchers have since estimated the brightness temperature of that white-light flare at roughly 8,800 to 10,900 Kelvin.2The Astrophysical Journal Letters. Magnitude Estimates for the Carrington Flare in 1859 September: As Seen from the Original Records The flare was accompanied by a coronal mass ejection, a vast bubble of magnetized plasma launched from the Sun at extraordinary speed, that reached Earth in under eighteen hours, a transit time far shorter than the typical two to three days.
At the surface, the effects were dramatic. A significant portion of the world’s roughly 200,000 kilometers of telegraph lines were disrupted, many of them unusable for eight hours or more, producing a real economic hit for the era’s communication infrastructure. The auroral display reached geomagnetic latitudes as low as about 18 degrees during the peak on September 2–3, meaning people in tropical regions like the Caribbean and Hawaii saw vivid red and green curtains of light in the sky.3PubMed Central. Duration and extent of the great auroral storm of 1859 Telegraph operators reported sparks leaping from their equipment, and some lines continued to transmit messages even after being disconnected from their batteries, powered by the electric currents the storm was driving through the wires.
How a Coronal Mass Ejection Becomes a Geomagnetic Storm
A solar flare is a burst of radiation. A coronal mass ejection, which often accompanies a large flare, is a physical cloud of plasma and embedded magnetic field that races outward from the Sun. The flare’s light arrives at Earth in about eight minutes and can disrupt radio communications. The CME, being made of matter, takes hours to days to arrive, and it is the CME that does the heavy lifting when it comes to geomagnetic storms.
Whether a CME actually hammers Earth’s magnetic field depends heavily on the orientation of its embedded magnetic field. Specifically, a strong southward-pointing magnetic component is the key driver, because a southward field can connect with and peel open Earth’s own northward-pointing magnetosphere, letting solar wind energy pour in.4Geophysical Research Letters. Coronal mass ejections, interplanetary ejecta and geomagnetic storms Faster CMEs tend to carry stronger magnetic fields, and the relationship is especially clear for very fast events traveling above about 1,200 kilometers per second.5Space Weather. Correlation between speeds of coronal mass ejections and the intensity of geomagnetic storms But a southward orientation is not the only path to trouble: researchers have documented cases where an intense geomagnetic storm occurred despite the CME’s magnetic rope being oriented at a low angle, with a strong azimuthal field component doing the damage instead.6The Astrophysical Journal Letters. Plasma and Magnetic Field Characteristics of Solar Coronal Mass Ejections in Relation to Geomagnetic Storm Intensity and Variability
Storm strength is measured by how far the horizontal component of Earth’s low-latitude magnetic field drops during the event, captured in a metric called the Dst index. The most intense storms, sometimes labeled superintense, push Dst below minus 500 nanotesla.7Extreme Events in Geospace. Supergeomagnetic Storms: Past, Present, and Future Estimates for the 1859 Carrington event place its Dst somewhere around minus 850 nanotesla or even lower, though direct measurements from that era are sparse.
How Likely Is a Repeat
The honest answer is that estimates vary widely depending on which statistical method and dataset you use, and that range itself tells you something about how uncertain the science still is. One analysis using extreme value theory on satellite-era X-ray flux data concluded that a Carrington-class flare is roughly a one-in-100-year event, and could be as frequent as one in 30 years under worst-case assumptions.8Space Weather. Using Extreme Value Theory for Determining the Probability of Carrington‐Like Solar Flares But a flare alone does not make a storm; the CME has to hit Earth, and its magnetic field has to be unfavorably oriented.
Looking at the geomagnetic side rather than the flare side, one study modeled the probability of another Carrington-class geomagnetic storm (Dst below about minus 850 nanotesla) occurring in the decade 2018–2027 at roughly 0.9%, with a 95% confidence interval stretching from about 0.5% to 1.9%.9Scientific Reports. Probability estimation of a Carrington-like geomagnetic storm A separate group using a different distribution found a considerably higher estimate: about 10% per decade if extreme storms follow a power-law distribution, or about 3% under a lognormal model.10Space Weather. Extreme geomagnetic storms: Probabilistic forecasts and their uncertainties The disagreement is not a sign that anyone is doing the math wrong; it reflects genuine uncertainty about the tail of the distribution, which is the part of the curve that governs the rarest, most extreme events and is, by definition, the part with the fewest data points.
Events That Were Almost as Bad, or Almost Hit Us
The Carrington event is not the only benchmark. Several storms since then have demonstrated that the Sun is perfectly capable of producing extreme events in the modern era. In March 1989, the largest magnetic storm of the twentieth century caused a cascading blackout across the Hydro-Québec power system in Canada, leaving millions without electricity.11Space Weather. A 21st Century View of the March 1989 Magnetic Storm In May 1921, a superstorm brought spectacular aurora and damaged telephone and telegraph systems associated with railroad operations across New York City and State, driven by a combination of strong local geomagnetic disturbance and the particular layout of the electrical networks of the day.12Space Weather. Intensity and Impact of the New York Railroad Superstorm of May 1921
The closest modern call came in July 2012. Solar active region 1520 launched a CME with an initial speed of roughly 2,500 kilometers per second, comparable to the fastest events on record. It crossed the distance from Sun to Earth’s orbital radius in only about 19 hours. The eruption was directed away from Earth, but NASA’s STEREO-A spacecraft, which happened to be in the CME’s path, recorded it in detail.13Space Weather. A major solar eruptive event in July 2012: Defining extreme space weather scenarios Researchers later used simulation models to estimate what would have happened if that CME had been aimed at Earth instead, and the results were sobering: the projected geomagnetic impact would have rivaled or exceeded the 1859 event.14Space Weather. Simulation of the 23 July 2012 extreme space weather event: What if this extremely rare CME was Earth directed? Earth dodged that particular bullet purely because of timing and geometry. The Sun rotates, and had the eruption occurred a week or so earlier, our planet would have been in the line of fire.
Ancient Storms Etched in Tree Rings
The instrumental record of geomagnetic storms only goes back about 150 years, which is a painfully short window for understanding how extreme these events can get. But nature keeps its own records. When intense bursts of solar energetic particles slam into Earth’s atmosphere, they produce a spike in the radioactive isotope carbon-14, which gets incorporated into tree rings. By measuring annual carbon-14 concentrations in ancient wood, scientists have identified several events that dwarf anything in the historical record.
The best-known of these is the 774 CE event, a globally coherent spike in carbon-14 detected in tree rings from both the Northern and Southern Hemispheres, confirming an extreme flux of high-energy solar particles.15Nature Communications. Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE Even more extreme spikes have been found at 7176 BCE and 5259 BCE, where atmospheric carbon-14 concentrations jumped by roughly 2% within two years, exceeding all previously known events including 774 CE.16PubMed Central. Tree-rings reveal two strong solar proton events in 7176 and 5259 BCE These ancient events did not cause geomagnetic storms in the way we usually talk about them since nobody was measuring Dst indices thousands of years ago, but they tell us the Sun is capable of eruptions far more powerful than anything in the observed record. If something on the scale of the 774 CE event happened today, the particle radiation alone would pose serious threats to satellites, astronauts, and potentially even aviation electronics, independent of any geomagnetic effects.
What a Carrington-Level Storm Would Do to Power Grids
The mechanism that turns a geomagnetic storm into a blackout is called geomagnetically induced current, or GIC. When Earth’s magnetic field fluctuates rapidly, it induces electric fields at the surface, and those fields drive quasi-DC currents through any long grounded conductor: power transmission lines, pipelines, railroad tracks, submarine cables.17Frontiers in Astronomy and Space Sciences. First direct observations of interplanetary shock impact angle effects on actual geomagnetically induced currents: The case of the Finnish natural gas pipeline system The biggest worry is high-voltage transformers. These currents push the transformer’s magnetic core into a half-cycle saturation, a condition the device was never designed to handle. Even comparatively low GIC levels, below 7 amps, have been observed to cause measurable increases in reactive power consumption in single-phase bank transformers, which have very little built-in tolerance.18Space Weather. Geomagnetically Induced Currents, Transformer Harmonics, and Reactive Power Impacts: Observed Response of a Single‐Phase Transformer in New Zealand
Under extreme pulse GIC conditions, the transient saturation process can generate harmonics, overheating, and potential permanent damage to transformer windings.19Electric Power Systems Research. Analysis of transformer transient saturation process characteristics under geomagnetically induced current pulses Large power transformers are custom-built, enormously expensive, and can take a year or more to manufacture and install. If a Carrington-level storm damaged a significant number of them simultaneously across a continent, the bottleneck would not be restoring power; it would be replacing hardware that cannot be quickly produced.
Economic modeling of this scenario suggests the direct costs from lost electricity are only about half the total damage. On average, indirect supply-chain disruptions, businesses losing revenue because their suppliers have no power, goods spoiling, manufacturing halting, roughly double the figure. If those indirect costs are ignored in planning, the true macroeconomic exposure is seriously underestimated.20Space Weather. Quantifying the daily economic impact of extreme space weather due to failure in electricity transmission infrastructure
Satellites, GPS, and Communications
Geomagnetic storms heat and expand Earth’s upper atmosphere, increasing the drag on satellites in low orbit. During the October 2003 superstorm, the orbital decay rate of a representative satellite at 400 kilometers altitude jumped from about 73 meters per day to 104 meters per day. During the May 2024 storm, which struck during a period when solar activity had already elevated baseline atmospheric density, the decay rate surged from about 155 meters per day to 258 meters per day.21Book of Abstracts and Contributed Papers / International scientific conference Meeting on Operational and Research Capabilities for Better Understanding Solar-Terrestrial Interactions. Thermospheric Response to Geomagnetic Storms and Impacts on Satellite Orbital Decay: A Comparative Analysis of the 2003 and 2024 Events That kind of increased drag shortens a satellite’s useful life and, in the worst case, can cause uncontrolled reentry. SpaceX lost dozens of newly launched Starlink satellites to exactly this mechanism during a moderate storm in February 2022, a vivid preview of what a truly extreme event could do to dense satellite constellations.
GPS accuracy takes a direct hit during geomagnetic disturbances. The ionosphere, the electrically charged layer of the upper atmosphere that GPS signals must pass through, becomes turbulent. This produces scintillation, rapid fluctuations in signal strength and phase. During severe storms, GPS receivers can lose lock on satellites entirely, particularly on the L2 frequency band, which is more vulnerable to deep signal fades.22Journal of Geophysical Research: Space Physics. GPS Scintillations and Losses of Signal Lock at High Latitudes During the 2015 St. Patrick’s Day Storm The May 2024 storm demonstrated that large position errors can occur even at mid-latitudes when strong scintillation and steep electron density gradients align.23Journal of Geophysical Research: Space Physics. Spatio‐Temporal Evolution of Mid‐Latitude GPS Scintillation and Position Errors During the May 2024 Solar Storm Even when ionospheric scintillation itself is not extreme, geomagnetic storms can degrade GPS positioning accuracy through increased cycle slips, measurement noise, and data outages.24Advances in Space Research. Analysis of global ionospheric scintillation and GPS positioning interference triggered by full-halo CME-driven geomagnetic storm: A case study
Submarine telecommunications cables face their own version of the problem. Modern fiber-optic cables carry data as light, which is immune to electromagnetic interference, but they also contain an internal copper conductor to supply electrical power to the repeaters spaced along the cable’s length. During a geomagnetic storm, induced voltages add to or subtract from this power feed, potentially pushing repeaters out of their operating range.25Space Weather. An Examination of Geomagnetic Induction in Submarine Cables Modeling work has confirmed that even current-generation cables remain vulnerable to space weather hazards through this mechanism.26Space Weather. Validating SCUBAS Predictions of Geomagnetically Induced Voltage in Submarine Cables Using Legacy Superstorm Observations Since the vast majority of intercontinental internet traffic travels through submarine cables, a severe storm could simultaneously degrade both satellite-based and cable-based communications.
What Happens to Aviation
Polar flight routes, commonly used for trans-Pacific and trans-Atlantic flights, are particularly exposed. During a solar proton event, high-energy particles funnel into the atmosphere over the poles along geomagnetic field lines, and the geomagnetic field does relatively little to shield these high latitudes.27Space Weather. Empirical determination of solar proton access to the atmosphere: Impact on polar flight paths This creates two problems. First, the bombardment degrades high-frequency radio communication, which is the primary means of air traffic control contact over oceanic and polar regions where line-of-sight VHF radio and radar coverage do not reach. Second, the energetic particles increase radiation exposure for passengers and crew at flight altitude. Airlines already reroute planes away from polar paths during major solar proton events, a procedure that adds hours of flight time and substantial fuel cost.
How Much Warning Would We Get
The warning timeline for a Carrington-level event is uncomfortably short. A solar flare’s radiation arrives at the speed of light, giving about eight minutes of notice for the electromagnetic burst. A fast CME, the kind that produced the 1859 event, covers the Sun-to-Earth distance in roughly 15 to 20 hours. But the critical question, whether the CME’s magnetic field is oriented to cause a severe storm, can only be answered when the cloud actually reaches monitoring spacecraft parked upstream of Earth.
Those spacecraft currently sit at the L1 Lagrange point, about 1.5 million kilometers sunward of Earth. For a fast CME, that provides on average only a few tens of minutes of definitive lead time, and for very fast solar wind speeds the advance warning drops to just minutes.28Space Weather. Monitoring the Solar Wind Before It Reaches L1 During the May 2024 G5 storm, researchers tested the idea of using the STEREO-A spacecraft, which happened to be positioned slightly closer to the Sun than L1, to squeeze out a bit more lead time. It helped, but the gains are still measured in minutes, not hours.
Computational models like WSA-ENLIL+Cone, which simulates CME propagation through the solar wind, are used operationally at space weather centers worldwide to forecast CME arrival times.29Journal of Space Weather and Space Climate. Verification of real-time WSA−ENLIL+Cone simulations of CME arrival-time at the CCMC from 2010 to 2016 Newer frameworks are working to model what happens when multiple CMEs interact in transit, a scenario that can amplify or reduce the eventual impact at Earth.30The Astrophysical Journal. Study of Evolution and Geo-effectiveness of Coronal Mass Ejection–Coronal Mass Ejection Interactions Using Magnetohydrodynamic Simulations with SWASTi Framework These models can predict roughly when a CME will arrive and how fast it will be, but they still cannot reliably forecast the magnetic field orientation, which is the single variable that matters most for storm intensity. That gap means the difference between “a strong storm is possible” and “brace for a historic storm” often cannot be communicated until the CME is practically at the doorstep.
What Utilities and Governments Are Doing About It
Mitigation strategies exist, even if they are not yet universally deployed. New Zealand offers a useful case study. Researchers worked directly with the national grid operator, Transpower, to develop a targeted plan of line disconnections during elevated GIC conditions. This approach has been adopted as an operational procedure in the national control room. Simulations showed that installing just 14 capacitor blocking devices at key transformers would reduce the total GIC flowing through the network by an additional 16%.31Space Weather. Geomagnetically Induced Current Mitigation in New Zealand: Operational Mitigation Method Development With Industry Input Those capacitor blockers are relatively inexpensive compared to replacing a damaged transformer, and Transpower is considering deploying them.
Elsewhere, the approach varies. Some countries with high geomagnetic latitude exposure, like Finland and Canada, have invested in real-time GIC monitoring networks and hardened transformer designs. Others, particularly in lower-latitude regions where GIC risk is perceived as smaller, have done less. The May 2024 storm, which was the strongest in over two decades, served as a wake-up call for many grid operators who had not previously experienced significant GIC impacts in their networks.
Birds, Migration, and Magnetic Confusion
One of the more surprising findings in space weather research involves migratory birds. Many species rely on Earth’s magnetic field for navigation during nocturnal migration. When geomagnetic storms scramble the field, the birds respond. A large-scale study using weather radar data across the United States found a 9 to 17% decrease in migration intensity during severe space weather events in both spring and fall, after controlling for atmospheric weather conditions. During fall migration, birds also showed reduced effort flying against the wind, suggesting they were drifting more passively rather than actively navigating. The effect was strongest when overcast skies combined with geomagnetic disturbance, removing both celestial and magnetic cues simultaneously.32PubMed Central. Space weather disrupts nocturnal bird migration
Experimental work with captive European robins confirmed the pattern at an individual level: the birds reduced their nocturnal migratory restlessness during simulated solar storms, then compensated by increasing activity in the early morning, as if shifting their migration into daylight hours when visual cues could substitute for the disrupted magnetic sense.33PubMed Central. Magnetic storms disrupt nocturnal migratory activity in songbirds A Carrington-level event, which would distort the magnetic field far more severely and across a much wider geographic area than any storm in the study period, could plausibly ground large numbers of migrating birds for its duration. What that would mean for migration timing, energy reserves, and survival is an open question nobody has been forced to answer yet.