Solar Storms and Space Weather Explained — The Threat From the Sun

The Sun is 150 million kilometres away. On most days that feels like a comfortable buffer. But the Sun is not a quiet, stable object sitting politely in the sky. It is a churning, magnetically violent star that regularly hurls billions of tonnes of charged plasma into space. When that plasma is aimed at Earth, we call it a solar storm, and the consequences range from spectacular auroras to collapsed power grids and ruined satellites.

Space weather is real, it is measurable, and the infrastructure of modern civilisation is more exposed to it than most people realise. This article explains where solar storms come from, how scientists rate them, what history tells us about the worst cases, and what can and cannot be done to protect against them.

Key takeaways

• Solar flares are intense bursts of electromagnetic radiation that travel at the speed of light and reach Earth in about eight minutes.
• Coronal mass ejections (CMEs) are slower clouds of magnetised plasma that usually take one to three days to arrive and cause the most serious geomagnetic disruption.
• NOAA rates space weather on three five-step scales: G (geomagnetic storms), S (solar radiation storms) and R (radio blackouts).
• The Carrington Event of 1859 is the benchmark extreme storm; it disrupted telegraphs worldwide and produced auroras in the tropics.
• The 13 March 1989 storm collapsed Hydro-Québec's grid in under two minutes and left millions in the dark for about nine to twelve hours.
• A Carrington-level event today could leave 20 to 40 million Americans without power for weeks to years, with costs of $0.6–2.6 trillion, according to a 2013 Lloyd's report.
• In May 2024, Earth saw its first G5 (extreme) geomagnetic storm since October 2003.

What is space weather?

Space weather is a catch-all term for the environmental conditions in near-Earth space driven by solar activity. Unlike terrestrial weather, which is governed by atmospheric pressure and temperature, space weather is governed by magnetic fields and energetic particles. It has four main drivers: solar flares, coronal mass ejections, solar energetic particle events, and high-speed solar wind streams from coronal holes.

All four ultimately come from the Sun's magnetic field. The Sun is a ball of plasma that rotates faster at its equator (about 25 days) than near its poles (more than 30 days). That differential rotation, combined with convection in the outer third of the star, twists and stretches magnetic field lines. Where bundles of field poke through the visible surface, they suppress the upward flow of heat and create cooler, darker patches: sunspots. The energy that powers the Sun in the first place comes from fusion in its core, which we cover in how the Sun works; space weather is what happens when some of that energy is stored in tangled magnetism and then released all at once.

The Sun operates on an approximately 11-year solar cycle of rising and falling magnetic activity. At solar maximum, sunspot counts peak and flare and CME frequency increases sharply. Solar Cycle 25 began in December 2019. On 15 October 2024, NASA, NOAA and the international Solar Cycle Prediction Panel announced that the Sun had entered its solar maximum period, while cautioning that the exact peak month can only be identified in hindsight. The strongest flare of the cycle up to that point, an X9.0, erupted on 3 October 2024. For a closer look at the current peak, see our explainer on solar maximum.

Solar flares — the speed-of-light punch

A solar flare is a sudden, intense release of energy above an active sunspot region, caused by magnetic reconnection: oppositely directed field lines snap together and reconfigure into a lower-energy state, converting stored magnetic energy into heat, particle acceleration and light. The energy is released across the spectrum, but the part that matters most for Earth is X-rays and extreme ultraviolet radiation. These travel at the speed of light and arrive in roughly 8 minutes and 20 seconds.

Flares are classified by their peak X-ray brightness as measured by NOAA's GOES satellites. Each letter is ten times stronger than the one before: A, B, C, M and X. Within a class, the number is a multiplier, so an X2 flare is twice as strong as an X1. C-class flares are minor. M-class flares are moderate and can cause brief radio blackouts on the sunlit side of Earth. X-class flares are the most intense.

The largest flare of the space age erupted on 4 November 2003. It saturated the GOES X-ray detectors, so it was first logged as X28; later analyses that used the flare's effect on very-low-frequency radio signals in the ionosphere put it at roughly X40 to X45. A week earlier, on 28 October 2003, an X17 flare had already caused radio and navigation problems.

The direct effect of a flare is fast. X-rays ionise the upper atmosphere on the dayside, which absorbs high-frequency (HF) radio waves that aircraft, ships and amateur operators rely on, and can degrade GPS signals within minutes. But a flare is over in minutes to hours, and its radiation cannot penetrate to the ground. Flares alone are rarely the biggest threat.

Coronal mass ejections — the slow freight train

A coronal mass ejection is a huge bubble of magnetised plasma expelled from the Sun's corona, often, though not always, alongside a flare. A large CME can carry on the order of a billion tonnes or more of material. CMEs travel at a few hundred to roughly 3,000 kilometres per second, so they take anywhere from under a day to about three days to reach Earth. That gives forecasters time to warn, if they spot the eruption and judge its direction correctly.

When a CME arrives, its magnetic field meets Earth's magnetosphere. If the CME's field points southward, opposite to Earth's northward-pointing field on the dayside, the two reconnect efficiently and energy pours into the magnetosphere. The result is a geomagnetic storm. Currents surge through the ionosphere and the ring of charged particles that circles Earth, and the magnetic field at the ground wobbles measurably. Those wobbles are what induce currents in long conductors such as power lines and pipelines. Our explainer on how magnetism works covers why a changing magnetic field drives a current in a conductor.

Fast CMEs also drive a shock wave ahead of them. That shock, and the flare itself, can accelerate protons to very high energies. These solar energetic particle (SEP) events can arrive within tens of minutes and are the main radiation hazard to astronauts and to satellite electronics.

How space weather is rated: NOAA's three scales

Forecasters needed a common language for how bad a storm is, so NOAA's Space Weather Prediction Center (SWPC) uses three five-step scales, each tied to a physical measurement. The table below summarises them using the figures SWPC publishes.

Scale	What it measures	Level 1 (minor)	Level 5 (extreme)	Level 5 frequency per 11-year cycle
G — geomagnetic storms	Planetary Kp index	G1 = Kp 5 (about 1,700 per cycle)	G5 = Kp 9	About 4 (4 days)
S — solar radiation storms	Flux of protons ≥10 MeV	S1 = 10 pfu (about 50 per cycle)	S5 = 100,000 pfu	Fewer than 1
R — radio blackouts	Peak X-ray flare class	R1 = M1 (about 2,000 per cycle)	R5 = X20	Fewer than 1

Source: NOAA SWPC, "NOAA Space Weather Scales". pfu = particle flux units.

The descriptions attached to the top levels are sobering. SWPC says a G5 storm can bring widespread voltage-control problems, blackouts and transformer damage; an S5 storm carries an unavoidable radiation hazard for astronauts on spacewalks and can permanently damage satellites; and an R5 event can black out HF radio across the whole sunlit hemisphere for hours.

The key point is that these are different phenomena that arrive on different clocks. A big eruption can produce an R event in eight minutes, an S event in under an hour, and a G event a day or two later.

The Carrington Event — the benchmark for disaster

On 1 September 1859, the English astronomer Richard Carrington was sketching sunspots when he saw two patches of intense white light flare up within the group. Another observer, Richard Hodgson, saw the same thing independently. It was the first recorded observation of a solar flare. About 17 to 18 hours later, an unusually fast CME struck Earth, and the strongest geomagnetic storm in the instrumental record began.

The effects were remarkable for the technology of the time. Telegraph systems across Europe and North America failed or misbehaved. Some operators reported shocks and sparks from their equipment; others found they could keep sending messages with their batteries disconnected, running on currents induced by the storm. Auroras were reported in the Caribbean and other tropical latitudes, and in the Rocky Mountains the glow was bright enough that some people reportedly got up thinking it was dawn.

Some popular accounts claim that Greenland ice cores record the Carrington Event as a spike in nitrate. Later work, notably a 2012 study by Eric Wolff and colleagues, argued that those nitrate spikes come from wildfire smoke and other non-solar sources, so ice cores are not a reliable record of this storm. The strongest evidence for its size comes from magnetometer records and aurora reports.

Carrington is not the only extreme case in the historical record. Researchers also study the May 1921 "New York Railroad" storm, which caused fires and outages in telegraph and railway signal systems, and they reconstruct older events from aurora records. And on 23 July 2012, a CME that NASA researchers rated as Carrington-class erupted from a region of the Sun facing away from Earth and swept past Earth's orbit, where NASA's STEREO-A spacecraft measured it. Daniel Baker and colleagues analysed the event in a 2013 paper, and Ying Liu, Janet Luhmann and colleagues later estimated that, had it erupted about nine days earlier, it would likely have hit Earth. For a scenario-based look at a modern repeat, see our companion piece on what a Carrington-type storm would do today.

1989, 2003, 2022 and 2024 — modern warnings

The modern record is already instructive.

March 1989. After a series of flares, a powerful CME hit Earth on 13 March 1989. Just after 2:44 a.m., geomagnetically induced currents tripped protective equipment on Hydro-Québec's network, and according to a NASA account, the entire Québec grid lost power in less than two minutes. Millions of people were left without electricity, the Montreal Metro was shut during the morning rush and Dorval Airport closed; most accounts give the outage as about nine hours, while NASA's account says twelve. In the United States, more than 200 grid problems were logged during the storm, though none caused a blackout, and a large step-up transformer at a New Jersey nuclear plant was damaged. NASA's TDRS-1 communications satellite logged more than 250 anomalies, and auroras were seen as far south as Florida and Cuba.

October–November 2003. The "Halloween storms" combined a string of X-class flares, several fast CMEs and strong radiation storms. Japan's ADEOS-II Earth-observation satellite was lost. Many spacecraft reported anomalies, the FAA's Wide Area Augmentation System, which improves GPS accuracy for aircraft, was unavailable for precision approaches for many hours, and a short blackout hit part of southern Sweden. Airlines rerouted polar flights to avoid radio blackouts and radiation exposure.

February 2022. On 3 February 2022, SpaceX deployed 49 Starlink satellites into a deliberately low orbit, about 200 km up, from which they would raise themselves to their operating altitude. A relatively modest geomagnetic storm the next day heated and expanded the upper atmosphere, raising drag. SpaceX said that up to 40 of the 49 would re-enter; a later peer-reviewed analysis put the loss at 38. It was a clear demonstration that you do not need an extreme storm to cause expensive damage. Our guide to how satellites stay in orbit explains why a thicker atmosphere at low altitude matters so much.

May 2024. On 9 May 2024, SWPC issued its first G4 (severe) storm watch since January 2005, warning of several Earth-directed CMEs launched from a sunspot region roughly 16 times the diameter of Earth. On the evening of 10 May, the storm reached G5, the first extreme geomagnetic storm since October 2003. Auroras were seen across much of Europe and as far south as Alabama and northern California in the United States. Farmers reported problems with GPS-guided tractors during planting season, HF radio and satellite navigation were degraded, and grid operators activated storm procedures. Major infrastructure failures were avoided, partly through preparedness and partly because the storm, while extreme by modern standards, was well short of Carrington.

Why the power grid is the weak point

To understand the grid risk, think about what a geomagnetic storm actually does at ground level. As the magnetic field fluctuates, it induces an electric field in the Earth's crust, typically measured in volts per kilometre. A high-voltage transmission line is a long conductor grounded at both ends through transformers. The induced electric field pushes a slowly varying, almost direct current through the line and the transformers' neutral connections. These are geomagnetically induced currents (GICs).

Transformers are designed for alternating current. A quasi-DC current offsets the magnetic flux in the transformer's iron core, pushing it into half-cycle saturation. A saturated transformer draws large amounts of reactive power, generates harmonics that can confuse protective relays, and heats up in places that were never designed to carry that load. In 1989, it was protective equipment reacting to harmonics and voltage drops, not physical destruction, that brought down the Québec network so quickly.

Several factors make a given grid more or less exposed:

• Latitude. Storm currents in the ionosphere are strongest under the auroral oval, so high-latitude regions such as Scandinavia, Canada and the northern United States see the largest effects. In extreme storms the oval expands toward the equator.
• Ground conductivity. Grids built on resistive igneous rock, like the Canadian Shield under Québec, see stronger induced electric fields than grids on conductive sediments.
• Line length and voltage. Long, high-voltage lines with low resistance carry more GIC.
• Coastlines. The contrast between conductive seawater and resistive land can concentrate induced fields near coasts.

The consequence that worries planners most is damage to large high-voltage transformers. These are custom-built, can weigh hundreds of tonnes, and take many months to manufacture and deliver. Utilities do not keep large stocks of spares. Losing a few at once could turn a short outage into a long one. If you want a primer on how the grid moves power in the first place, see how electricity works.

What a Carrington-level event would do today

The 2013 Lloyd's of London and Atmospheric and Environmental Research report Solar Storm Risk to the North American Electric Grid estimated that a Carrington-scale storm could leave 20 to 40 million people in the United States at risk of extended power outages lasting from 16 days to one to two years, depending largely on how many transformers were damaged and how fast replacements arrived. It put the economic cost at $0.6 trillion to $2.6 trillion. The report identified the Atlantic corridor between Washington, D.C. and New York City as the most at-risk area when weighted by population, and suggested return periods of roughly 50 years for Québec-level storms and 150 years for very extreme storms.

These numbers are model estimates, not predictions, and they depend heavily on assumptions about transformer vulnerability and grid operator response. Since 2013, North American grid rules have also changed, which we discuss below. But the basic shape of the risk is not controversial: a rare, extreme storm could cause damage that takes far longer to repair than a hurricane or ice storm, because the broken parts are so hard to replace.

Beyond the grid, an extreme storm would threaten several other systems at once:

• Satellites. Energetic particles can upset or destroy electronics, and surface and internal charging can cause damaging electrical discharges. Increased drag can drag low satellites down early and, as in 2022, destroy those in low parking orbits. Thousands of tracked objects would also shift position, temporarily degrading collision-avoidance catalogues.
• GPS and timing. Ionospheric disturbances scramble the signals that GPS receivers depend on, reducing accuracy or causing loss of lock. Because many networks, from cellular systems to financial trading, take their timing from GPS, the effects reach well beyond navigation. Our explainer on how GPS satellites know your location shows why the signal is so sensitive to delays.
• Aviation. Polar routes depend on HF radio for communication. During radiation storms, airlines reroute to lower latitudes, costing fuel and time.
• Pipelines and railways. Long metal pipelines and railway signalling circuits also pick up induced currents, which can accelerate corrosion or cause signalling errors.
• Astronauts. Crews outside Earth's protective magnetosphere face the worst exposure from SEP events.

How we detect and predict space weather

Forecasting begins with watching the Sun. NASA and ESA's SOHO observatory, launched in 1995, carries coronagraphs that block the bright disc so faint CMEs can be seen leaving the Sun. NASA's Solar Dynamics Observatory images the surface in multiple wavelengths, and STEREO-A views the Sun from a different angle, which helps judge whether a CME is heading toward Earth. NASA's Parker Solar Probe is flying closer to the Sun than any spacecraft before it to study how the solar wind and energetic particles are accelerated.

The final warning comes from spacecraft parked at the L1 Lagrange point, about 1.5 million kilometres sunward of Earth, including NOAA's DSCOVR and the newer Space Weather Follow On–Lagrange 1 (SWFO-L1). Our article on Lagrange points explains why that spot is so useful. These craft sample the solar wind before it reaches us. When a CME passes L1, forecasters typically have roughly 15 to 60 minutes before it reaches Earth's magnetosphere, depending on its speed.

NOAA's Space Weather Prediction Center in Boulder, Colorado issues forecasts, watches, warnings and alerts around the clock. The UK Met Office runs its own operational centre, MOSWOC, and ESA operates a space weather service network. Forecasters combine coronagraph images with computer models such as WSA–Enlil, which simulate how a CME travels through the solar wind, to estimate arrival times. Typical arrival-time errors are still several hours.

The weakest link is the orientation of the CME's magnetic field, usually discussed as its north–south component, Bz. A strongly southward (negative) Bz produces a big storm; a northward one may produce little. Today, Bz cannot be reliably measured until the cloud reaches L1, which is why forecasters can say "a CME is coming tomorrow" with some confidence but can only say "and it will be severe" less than an hour before impact.

What protection exists

Protection is real but uneven.

Grid operators in North America work under NERC reliability standards developed after a 2013 FERC order. Utilities must have operating procedures for geomagnetic disturbances and assess whether their equipment can withstand a defined benchmark storm. During a storm warning, operators can reduce transfers on vulnerable lines, bring extra generation online to supply reactive power, postpone maintenance and avoid running equipment near its limits. Some utilities have installed devices that block DC from flowing through transformer neutrals, and series capacitors that block GIC on certain lines. Hydro-Québec itself made extensive changes after 1989, including series compensation on its long lines.

Satellite operators use radiation-hardened parts, shielding and fault-tolerant software, and they can put spacecraft into safe mode, delay launches or postpone manoeuvres when a storm is forecast. The 2022 Starlink loss pushed operators of large constellations to pay much closer attention to forecasts before deploying satellites into low orbits.

Astronauts aboard the International Space Station shelter in the more heavily shielded parts of the station during strong radiation storms. The ISS still sits well inside Earth's magnetosphere, which provides much of its protection. Crews travelling to the Moon will not have that shield, so mission planners for Artemis are designing shelter procedures and onboard radiation monitoring.

Common misconceptions

"A solar storm could hurt people on the ground." Not directly. Earth's atmosphere and magnetic field stop the radiation and particles. The risk to people on the ground comes indirectly, through failures in infrastructure such as power and communications.

"Auroras far south mean disaster." Low-latitude auroras show that a storm is strong, and they are a good reason for grid and satellite operators to be alert. But May 2024 produced auroras over much of the world without major infrastructure failures.

"Solar storms could wipe out the internet." Fibre-optic cables carry light, which induced currents do not affect. The concern is the electrically powered repeaters along long undersea cables and the power supply to data centres. A worst case could cause regional outages, but the claim of a global "internet apocalypse" goes well beyond the evidence.

"Earth's magnetic field is our only defence." The magnetic field deflects most charged particles, and the atmosphere absorbs the X-rays. A weaker field would let more energy in, which is part of what makes scenarios like Earth's magnetic field flipping interesting, but even during reversals the atmosphere still blocks harmful radiation at ground level.

Open questions

Several questions remain open. Forecasters cannot yet predict when a sunspot region will flare, only estimate the odds. The orientation of a CME's magnetic field cannot be measured until the cloud is nearly at Earth. And the true frequency of Carrington-class storms is uncertain because the instrumental record covers fewer than two centuries. Studies of tree rings have also found sharp spikes in carbon-14, such as one in the year 774–775 CE, which many researchers attribute to extreme solar particle events far larger than anything seen in the modern era. Whether storms like those could happen in our lifetimes is one of the central questions of the field.

FAQ

Q: What is the difference between a solar flare and a coronal mass ejection? A: A flare is a burst of light and radiation that reaches Earth in about eight minutes and mainly disrupts radio. A coronal mass ejection is a cloud of magnetised plasma that takes one to three days to arrive and causes geomagnetic storms that affect power grids and satellites.

Q: How often do extreme G5 geomagnetic storms happen? A: NOAA estimates about four G5 storm days per 11-year solar cycle. The May 2024 storm was the first G5 since October 2003, which shows how much the actual rate can vary from cycle to cycle.

Q: Can a solar storm cause a blackout? A: Yes. On 13 March 1989, geomagnetically induced currents caused Hydro-Québec's grid to collapse in under two minutes, leaving millions of people without power for about nine to twelve hours.

Q: How much warning do we get before a solar storm hits? A: Coronagraphs can spot a CME leaving the Sun one to three days before arrival. The crucial detail, the orientation of its magnetic field, is only measured when it passes L1, giving about 15 to 60 minutes of warning.

Q: Are solar storms dangerous to astronauts? A: Strong solar radiation storms are a real hazard, especially outside Earth's magnetosphere. ISS crews shelter in better-shielded areas during strong events, and lunar missions are planning dedicated shelter procedures.

Q: What was the Carrington Event? A: It was the extreme geomagnetic storm of early September 1859, which followed a flare observed by Richard Carrington on 1 September. It disrupted telegraph systems across Europe and North America and produced auroras at tropical latitudes.

The bottom line

The Sun is the engine of our solar system, and it occasionally reminds us that engines can misfire. Solar storms are not rare anomalies; they are routine features of a star with an active magnetic cycle. What is new is our exposure. Each decade we put more satellites into orbit, extend power lines over longer distances, and tie more critical systems to GPS timing, all of which raise the stakes of the next big storm.

The Carrington Event was a warning issued in 1859 to a world that lit its homes with oil and gas. The May 2024 storm was a stress test that went relatively well. The tools to harden infrastructure exist, and grid rules have improved since 1989. The open question is whether that investment keeps pace with our growing dependence on the systems most exposed to the Sun.

Sources

• NOAA Space Weather Prediction Center, NOAA Space Weather Scales
• NASA (via ScienceDaily), The Day the Sun Brought Darkness, March 2009
• NASA Scientific Visualization Studio, NASA, NOAA Announce That the Sun Has Reached the Solar Maximum Period, October 2024
• UK Space Agency, Recently launched Starlink satellites hit by geomagnetic storm, February 2022
• Lloyd's / Atmospheric and Environmental Research, Solar Storm Risk to the North American Electric Grid (2013)

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