The Carrington Event was the most intense geomagnetic storm in recorded history, triggered by a massive solar eruption that struck Earth in September 1859. It lit up the night sky with auroras visible as far south as the tropics, sent electric currents surging through telegraph wires, and left instruments around the world behaving erratically for hours. Whether something comparable could happen again is not a matter of “if” but “when,” and the consequences for a civilization now wired with power grids, satellites, and GPS would be far more serious than anything a telegraph operator faced.
What Happened in September 1859
On the morning of September 1, 1859, the English astronomer Richard Carrington was sketching sunspots through his telescope when a brilliant white-light flare erupted from the solar surface. From his original drawings, researchers have estimated the flaring region reached a blackbody brightness temperature of roughly 8,800 to 10,900 K, an extraordinary burst of radiant energy from a relatively compact area of the Sun.1The Astrophysical Journal Letters. Magnitude Estimates for the Carrington Flare in 1859 September: As Seen from the Original Records A massive cloud of magnetized solar plasma, known as a coronal mass ejection, followed, crossing the roughly 150 million kilometers to Earth in under 18 hours. Typical solar storms take two to four days to make the same trip.
When that plasma slammed into Earth’s magnetic field, the results were spectacular and disruptive. Auroras were described as a deep crimson so bright “one could read a newspaper by” the light, and they were visible down to geographic latitudes of about 25 degrees, regions where auroras are essentially never seen.2Advances in Space Research. Eyewitness reports of the great auroral storm of 1859 The world’s roughly 125,000 miles of telegraph lines were hammered with induced currents. Many lines became unusable for eight hours or more, and operators in the Boston area famously disconnected their batteries and continued sending messages powered solely by the electric currents the storm was driving through the wires.3Advances in Space Research. The super storms of August/September 1859 and their effects on the telegraph system Other operators reported sparks jumping from their equipment, and in some cases telegraph paper caught fire.
How Intense Was the Storm, Exactly
Scientists gauge the strength of a geomagnetic storm by how much it depresses Earth’s horizontal magnetic field, measured in nanotesla (nT). The index used for this, called Dst, drops sharply negative during big storms. For the 1859 event, the only surviving magnetogram came from Bombay, India, and it recorded an extraordinary excursion of about −1,600 nT.4Advances in Space Research. Dst of the Carrington storm of 1859 For comparison, the March 1989 storm that blacked out the Hydro-Québec power grid in Canada reached roughly −590 nT. Researchers have debated whether the Bombay reading was entirely from magnetospheric currents or was partly boosted by ionospheric currents overhead. If the full value reflects the magnetospheric response, the Carrington Event stands essentially alone in the historical record.
Using extreme value statistical methods on solar X-ray flux data, one study estimated that a Carrington-class flare is roughly a once-in-100-year event, with a worst-case estimate putting it closer to once in 30 years.5Space Weather. Using Extreme Value Theory for Determining the Probability of Carrington‐Like Solar Flares An independent analysis reached a broadly consistent conclusion, estimating the expected return time for a Carrington-class event at about 110 years.6Space Weather. Extreme Value Analysis of Solar Flare Events So we are, statistically speaking, roughly overdue.
How Likely Is a Repeat in the Near Future
Pinning down probability for rare events is tricky, and published estimates vary by an order of magnitude. A 2019 study using a different statistical approach, focused on the geomagnetic disturbance itself rather than the flare size, estimated the probability of a Carrington-level storm (defined as reaching −850 nT or beyond) occurring in any given decade at just under 1%, with a 95% confidence range of about 0.5% to 1.9%.7Scientific Reports. Probability estimation of a Carrington-like geomagnetic storm That is far lower than the roughly 12% per-decade estimate that circulated in earlier analyses. The gap exists because different researchers use different storm metrics, different historical baselines, and different statistical frameworks. But even the conservative estimate means that over a human lifetime, you have a nontrivial chance of living through one.
There is also evidence that the Sun can do worse. Cosmogenic isotopes preserved in tree rings and ice cores have revealed signatures of extreme solar particle events far more powerful than anything recorded by instruments. The 774 CE event, for example, left a spike in carbon-14 that dwarfs modern observations, and even larger events have been identified at 993 CE, 660 BCE, and 7176 BCE, with estimated magnitudes tens of times larger than the biggest modern solar particle event.8Journal of Space Weather and Space Climate. No signature of extreme solar energetic particle events in high-precision 14C data from the Alaskan tree for 1844–1876 CE The Carrington Event itself, interestingly, left no detectable tree-ring signature, which tells researchers it was a severe magnetic storm but not necessarily accompanied by an extreme solar particle event.
Superflares on Sun-Like Stars
One way to gauge how extreme the Sun’s eruptions can get is to look at other stars of the same type. Data from the Kepler space telescope, which monitored tens of thousands of stars, revealed 365 superflares on solar-type stars in just 120 days of observation, including some on slowly rotating stars broadly similar to the Sun.9Nature. Superflares on solar-type stars Follow-up work extended this to over 1,500 superflares, confirming that the occurrence rate follows a power-law distribution remarkably similar to what we see for ordinary solar flares, just extended to higher energies.10The Astrophysical Journal Supplement Series. SUPERFLARES ON SOLAR-TYPE STARS OBSERVED WITH KEPLER. I. STATISTICAL PROPERTIES OF SUPERFLARES For stars closely matching the Sun’s temperature and slow rotation, superflares with energies around a hundred to a thousand times stronger than the Carrington Event appeared to occur roughly once every 800 to 5,000 years.
A more recent analysis using updated Kepler data identified nearly 2,900 superflares on Sun-like stars and found that superflares above a certain energy threshold happen about once per century on average for stars with Sun-like temperature and variability.11PubMed. Sun-like stars produce superflares roughly once per century The flare frequency-energy distribution matched what you would get by extrapolating the Sun’s known flare distribution upward, suggesting the same underlying mechanism produces both everyday flares and extreme events. That is not exactly comforting: it means the Sun is not inherently incapable of superflares; it just has not produced one while we have been watching with modern instruments.
What a Carrington-Scale Storm Would Do to the Power Grid
In 1859, the only electrical infrastructure in the world was telegraph wire. Today, the concern is high-voltage power transformers. When a geomagnetic storm drives currents through the ground, those geomagnetically induced currents (GICs) enter the grid through transformer ground connections. GICs push the magnetic cores of transformers into saturation, which causes voltage instability, harmonic currents flooding the network, and severe internal heating that can permanently damage the transformer.12Journal of Applied Research and Technology. Core saturation effects of geomagnetic induced currents in power transformers The cascade of effects includes malfunction of transmission line protection systems, reactive power surges, and voltage collapse across wide areas of the network.13Academic Press. Geomagnetically Induced Currents: A Threat to Modern Power Systems
The best-known modern example remains the March 1989 storm that knocked out the Hydro-Québec system, leaving six million people without power for nine hours. That storm was strong but nowhere near Carrington-class. Simulations of a Carrington-level storm on the 500 kV power grid in Guangdong, China, concluded that the GIC risk would be “very high” based on comparison with the currents responsible for the 1989 outage.14Space Weather. Simulation of Geomagnetically Induced Currents in a Low‐Latitude 500 kV Power Network During a Solar Superstorm Even low-latitude grids, which are sometimes assumed to be safer because GICs are strongest near the poles, turn out to be vulnerable.
High-voltage transformers are the particular chokepoint because they are expensive, custom-built, and take months or years to replace. If a superstorm damaged dozens simultaneously across a continent, the queue for replacements would dwarf manufacturing capacity. Induced currents also affect pipelines, where they accelerate corrosion and interfere with the electrical surveys used to monitor corrosion-prevention systems.15Advances in Space Research. The effects of geomagnetic disturbances on electrical systems at the Earth’s surface
The Economic Damage Is Larger Than Just the Blackout
Estimates of the cost of a Carrington-class event vary enormously depending on assumptions about the size of the blackout zone and how long power stays off. One important finding that applies across most models is that the direct cost of lost electricity represents only about half the total economic damage. The rest comes from indirect supply-chain disruption as industries that depend on power, communications, and transportation are knocked offline in sequence.16Space Weather. Quantifying the daily economic impact of extreme space weather due to failure in electricity transmission infrastructure If planners only account for the cost of undelivered electricity, they are missing roughly half the picture.
A detailed risk assessment for the United Kingdom estimated that a Carrington-scale event with no advance warning could cost the UK economy as much as £15.9 billion. With current space weather forecasting capability, that figure drops to about £2.9 billion, and with enhanced forecasting investments it could fall to around £0.9 billion.17PubMed Central. A Risk Assessment Framework for the Socioeconomic Impacts of Electricity Transmission Infrastructure Failure Due to Space Weather: An Application to the United Kingdom That is a single mid-sized country. Global estimates range into the trillions of dollars, though the figures are necessarily speculative because no one has ever stress-tested modern civilization against a storm this large.
Satellites, GPS, and the May 2024 Storm
Even storms well below Carrington strength cause real problems for the growing population of satellites in low Earth orbit. When a geomagnetic storm heats the upper atmosphere, it expands, and the increased air density at orbital altitudes drags satellites downward.18Space Weather. Satellite Orbital Drag During Magnetic Storms The most vivid recent example came in February 2022, when SpaceX launched 49 Starlink satellites into a geomagnetic storm. Atmospheric density at their deployment altitude of 210 km was at least 20 to 30% higher than normal, and 38 of the 49 satellites reentered and burned up within days.19Space Weather. The Thermosphere Is a Drag: The 2022 Starlink Incident and the Threat of Geomagnetic Storms to Low Earth Orbit Space Operations
The May 2024 storm, the strongest geomagnetic event in over two decades, offered a broader preview. Thousands of satellites experienced rapid altitude drops, and in some cases the increased drag caused satellites to tumble in uncontrolled ways. The number of satellites performing orbit-correction maneuvers surged from about 300 before the storm to nearly 5,000 on May 11. Collision warnings for UK-licensed operators spiked by 35% compared to the previous month. Meanwhile, ionospheric disruption caused GPS positioning errors of around 60 meters across much of the Americas during the storm’s peak hours.20Royal Society Open Science. The May 2024 geomagnetic storm: UK experience and perspective For systems that depend on centimeter-level precision, like autonomous farming equipment and aircraft landing aids, errors of that magnitude are disabling.
Beyond orbital drag, energetic particles during solar storms can flip bits in satellite electronics, a phenomenon called a single event upset. During periods of intense solar proton flux, one satellite in polar low Earth orbit detected a dramatically increased rate of these errors on two of its onboard computers, occurring only after the high-energy proton flux rose to roughly a thousand times above background levels.21Advances in Space Research. The connection between space weather and Single Event Upsets in polar low earth orbit satellites A Carrington-class event could multiply these effects across entire constellations simultaneously.
Radiation Risk for Air Travel
Earth’s atmosphere and magnetic field shield the surface from most solar particle radiation, but the shielding weakens at high altitudes and near the poles, exactly where long-haul flights spend much of their time. During the May 2024 storm, radiation sensors aboard a commercial flight measured elevated absorbed dose rates. The airline had already rerouted the flight to lower latitudes as a precaution, and analysis afterward suggested that without the deviation, crew exposure could have been up to three times higher.22Journal of Geophysical Research: Space Physics. Enhanced Radiation Exposure of Airline Crew and Passengers During the May 2024 Geomagnetic Storm
For a Carrington-scale event the concern escalates considerably. Modeling of known extreme events shows that the strongest modern ground-level enhancement, which occurred in February 1956, could deliver a total radiation dose at cruise altitude of about 5 millisieverts in polar regions, roughly 60 to 100 times the normal cosmic-ray dose for a 10-hour flight. The most powerful historical event identified in ice cores, the 7176 BCE event, could theoretically deliver about 225 millisieverts at cruise altitude in polar airspace, which exceeds the annual occupational exposure limit in a single flight.23Journal of Space Weather and Space Climate. Assessment of impacts to aviation radiation by extreme space weather events and new atmospheric radiation scales At mid-latitudes, geomagnetic shielding cuts these figures by more than an order of magnitude, which is why rerouting away from the poles matters so much. The same analysis found that avionics error rates from particle hits could spike by a factor of 10 to 1,000 during modern-scale events, and by up to 200,000 times during the most extreme historical events.
Early Warning and What It Buys You
The good news is that we are not flying blind the way Carrington was in 1859. Space weather forecasting now relies on a combination of remote solar observation and real-time in situ measurements from probes stationed at the L1 Lagrange point, about 1.5 million kilometers sunward of Earth.24Space Weather. Monitoring the Solar Wind Before It Reaches L1 When a coronal mass ejection passes L1, it typically reaches Earth 15 to 60 minutes later, depending on its speed. That is not a lot of time, but it is enough for grid operators to take protective action if they have pre-planned procedures in place.
Forecasting from farther out, using coronagraph imagery to track a CME as it leaves the Sun, offers hours to days of notice but carries significant timing uncertainty. Simulations of CME arrival times at one forecasting center over a six-year period showed meaningful errors, a challenge that could be reduced by adding a coronagraph viewpoint from the L4 or L5 Lagrange points, which sit along Earth’s orbit 60 degrees ahead of or behind the planet.25Journal 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 The European Space Agency’s Vigil mission, planned for the L5 point, aims to provide exactly this kind of side-angle view. But the UK risk assessment noted that existing monitoring satellites are aging, and without continued investment, forecasting capability will degrade rather than improve.
Hardening the Grid
Forecasting alone does not protect infrastructure; grid operators also need concrete plans for what to do with a warning. New Zealand offers one of the more detailed case studies. Working with its national grid operator, researchers developed a mitigation strategy based on targeted disconnection of specific transmission lines during a storm. The approach reduced the average 60-minute mean GIC at 27 of the 30 most at-risk transformers and cut total network GIC by 16%, all while maintaining continuous power supply across the country. On top of that, simulations showed that installing capacitor blocking devices at just 14 key transformers could reduce total network GIC by an additional 16%.26Space Weather. Geomagnetically Induced Current Mitigation in New Zealand: Operational Mitigation Method Development With Industry Input The strategy has been adopted as an operational procedure in the national control room.
This kind of targeted preparation matters because the alternative, losing transformers that take a year or more to replace, would extend a blackout from hours into months. Countries at higher geomagnetic latitudes, like Canada and the Nordic states, have invested more heavily in GIC monitoring and blocking devices. Others, especially at lower latitudes where the risk was historically assumed to be small, are only beginning to plan.
How Solar Storms Form in the First Place
The Sun’s magnetic field is not uniform or static. It concentrates in active regions, areas of intense and tangled magnetic flux that appear as sunspot groups. When the stored magnetic energy in an active region exceeds what the field configuration can sustain, it releases violently as a flare and, often, a coronal mass ejection. Larger and more magnetically complex active regions are capable of producing faster CMEs, but researchers have found only a weak positive correlation between active region size and individual CME speed.27The Astrophysical Journal. Dependence of Coronal Mass Ejection Properties on Their Solar Source Active Region Characteristics and Associated Flare Reconnection Flux A single eruption often involves only a small fraction of the region’s total energy, so a complex sunspot group might produce a string of eruptions of varying intensity rather than one definitive blast.
This unpredictability extends to CME structure. Recent simulations have shown that when multiple active regions interact through overlying magnetic structures, the resulting CME can develop a highly complex, tangled internal magnetic field, especially during solar maximum when active regions crowd the solar disk.28The Astrophysical Journal. The Birth of a Major Coronal Mass Ejection with Intricate Magnetic Structure from Multiple Active Regions Whether a CME’s magnetic field happens to point southward when it arrives at Earth, which is the orientation that most effectively breaches our planet’s magnetic shield, is still difficult to predict more than an hour or so in advance. This is the single biggest source of false alarms and missed calls in space weather forecasting: a fast, massive CME aimed straight at Earth can still produce a modest storm if its magnetic field points the wrong way.
Birds, Bees, and Magnetic Navigation
Geomagnetic storms do not just rattle human infrastructure. Many migratory animals use Earth’s magnetic field as a navigational compass, and strong disturbances appear to throw them off course. A study using weather radar data across the continental United States found a 9 to 17% decrease in nocturnal bird migration intensity during severe space weather events, in both spring and fall seasons. During autumn specifically, birds showed less effort flying against the wind, suggesting they drifted passively rather than actively navigating. The effect was worst when geomagnetic disturbance coincided with overcast skies, presumably because clouds hid the stars that serve as a backup navigational reference.29PubMed Central. Space weather disrupts nocturnal bird migration
A separate analysis of bird banding records across North America found that geomagnetic disturbance was associated with significantly increased vagrancy, meaning birds turning up far from their expected range, in the fall migration season. For a typical species, an increase in geomagnetic disturbance to two standard deviations above average conditions was linked to a 24% increase in vagrancy and a 250% spike in the number of spatiotemporally rare sightings.30Scientific Reports. Geomagnetic disturbance associated with increased vagrancy in migratory landbirds The sensitivity was widespread, affecting the majority of the 150 species in the dataset. A Carrington-class event, coinciding with peak fall migration, could theoretically produce an unusual burst of disoriented birds turning up in unexpected places, a phenomenon birders might notice even if they knew nothing about space weather.