Could a giant solar flare knock out the power grid?
— In the biggest solar storm on record, telegraph wires started working by themselves
Yes — it's happened before. In 1989, a solar storm knocked out power for about six million people in Quebec, Canada. The strange part: the sun's electricity never actually arrived. When Earth's magnetic field is shaken, the ground itself develops a faint electrical "push". It's long wires, stretching hundreds of kilometres, that end up collecting it all.
One night in September 1859. At a telegraph office in the United States, an operator was baffled. The equipment was clattering on its own, and sparks were flying off the paper, leaving a scorched smell.
On a hunch, he disconnected the battery. And the telegraph kept working — messages still got through to distant stations, battery or no battery. At the same time, red auroras were painting skies far to the south, places that never normally see them.
This was the largest solar storm ever recorded: the Carrington Event. If the same thing happened today, what would it mean for our lives?
Only two things make a solar eruption affect wires on the ground
When a huge eruption occurs on the Sun's surface, it hurls out an enormous cloud of electrically charged gas. When this slams into Earth, the magnetic field wrapping our planet (geomagnetic field) swings wildly for hours at a time.
When a magnetic field changes, it creates a push on electric charge around it. On the ground, this is only a few volts per kilometre — extremely weak. But a wire stretching hundreds of kilometres adds that push up from one end to the other.
In other words, the sun's electricity doesn't leap into the wire. It's Earth's shaking magnetic field that makes electricity "well up" inside long wires on the ground. Let's look at this step by step.
The solar cloud arrived many times faster than usual
The Sun constantly blows out a stream of gas called the "solar wind". It normally travels at about 400 km per second, taking roughly four days to reach Earth.
But during a major eruption, a much denser, faster cloud of gas bursts out all at once. This is called a "coronal mass ejection". The 1859 cloud is thought to have reached Earth in only about 17.6 hours after the eruption — working out to a speed of over 2000 km per second.
This cloud carries its own magnetic field. If that field points opposite to Earth's, the two link up, and the cloud's energy pours into the space around Earth. This large swing in the geomagnetic field is called a "geomagnetic storm". It's also when auroras spread far to the south (see Why can auroras only be seen near the North and South Poles? for how auroras work).
The longer the wire, the bigger the voltage it generates
When a magnetic field changes over time, it creates a push on electric charge around it — an electric field. This is the same principle a generator uses, called "electromagnetic induction". During a geomagnetic storm, this faint electric field spreads across the ground beneath our feet.
In a major geomagnetic storm, the ground's electric field reaches roughly a few volts per kilometre — about as weak as a single AA battery. But a wire adds up that voltage along its entire length. Try adjusting the dial in Figure 1 to change the wire's length.
Looking at the bottom row of Figure 1, you can see that almost nothing happens to the short wiring inside a house. The danger lies in long transmission lines strung across countries or states. Old telegraph wires were exactly this kind of long wire — which is thought to be why messages could get through in 1859 without any battery at all.
Isn't 2000 volts small compared to a power line?
Power transmission lines normally carry hundreds of thousands of volts. Next to that, 2000 volts looks tiny. The real problem is a difference in the "kind" of electricity.
The electricity in power lines is "alternating current" (AC), reversing direction dozens of times per second. The electricity generated by a geomagnetic storm, though, changes slowly over many minutes, so it behaves almost like one-directional "direct current" (DC). Substation transformers are built for AC, and when DC mixes in, the iron core gets magnetically "saturated" — filled up as a magnet. The transformer then overheats, and its ability to hold a steady voltage breaks down.
In March 1989 in Quebec, protective devices tripped one after another in this way, and the grid is reported to have shut down within about 90 seconds. It took about nine hours to restore power.
During the major geomagnetic storm of May 2024, red auroras were captured on camera in places like Hokkaido. This is called a "low-latitude aurora". The 1859 solar storm is estimated to have been even stronger — and it struck about 18 hours after Carrington first observed the eruption through his telescope.
During a geomagnetic storm, the layers of air high above Earth get disturbed too. This can throw off the satellite signals smartphones use for location, and make shortwave radio harder to receive. Satellites themselves can even lose altitude as they plough into the puffed-up atmosphere.
Summary
A cloud of gas blasted out by a giant solar eruption can violently shake Earth's magnetic field. As that field changes, it creates a faint electric field on the ground — only a few volts per kilometre. Transmission lines stretching hundreds of kilometres add that up and feed DC-like current into substation transformers. That's how a solar storm can end up causing blackouts on the ground. Today, countries around the world keep watch on the Sun and run systems to warn us before a storm arrives — this is called space weather forecasting. All we can do is prepare the same way we would for any ordinary power outage.
It isn't the Sun's electricity arriving.
A shaken geomagnetic field makes electricity well up inside long wires.
How electricity surges into wiring during a lightning strike is explained in Should you unplug things when thunder strikes?, and why Earth itself is a magnet is explained in Why does a compass needle point north?.
- Open the Space Weather page from Japan's National Institute of Information and Communications Technology (NICT) and check today's solar activity and geomagnetic conditions.
- The Japan Meteorological Agency's Geomagnetic Observatory page issues a "geomagnetic storm alert" whenever one occurs. When it's up, you can see the geomagnetic graph swinging wildly.
- On a night when a major geomagnetic storm is reported, go somewhere with little city light and take a long-exposure photo of the northern sky with your phone. Even if you can't see anything with your eyes, red colour sometimes shows up in the photo.
It usually takes one to three days for a solar eruption's effects to reach Earth. Having time to watch the forecast before waiting for the sky is part of what makes space weather so interesting.
Want to know more? — Terms, formulas, and textbook connectionsClearly labelled by level, from junior-high science to university-level courses
- JHSCovered in junior high school science
- HSCovered in high school "Physics" or "Earth Science"
- HS+Advanced high school content, or textbook sidebar material
- UnivNot covered in high school — university-level specialist courses (electromagnetism, space physics)
- ResearchNot yet settled even at university level — things researchers are actively investigating
JHSTerminology: this phenomenon has names
- Coronal mass ejection: an event where electrically charged gas bursts out from the Sun's outer atmosphere (the corona) as a coherent cloud.
- Geomagnetic storm: a major disturbance of Earth's magnetic field (geomagnetic field), caused by gas from the Sun, lasting from hours to days.
- Electromagnetic induction: the phenomenon where a changing magnetic field generates a voltage that drives current around it. Taught in second-year junior high science.
- Geomagnetically induced current: current that flows into long conductors — transmission lines, pipelines, and the like — during a geomagnetic storm.
JHSHSCheck with a formula: how many volts does a 1000 km wire generate?
Assuming the ground's electric field is uniform, let's calculate the voltage generated between the two ends of a wire. We'll also work out how fast the 1859 gas cloud was travelling.
| In symbols | V = E × L |
| In words | Voltage generated in the wire = ground electric field (volts per km) × wire length |
| Where it comes from | Faraday's law of electromagnetic induction. As the geomagnetic field changes over time, it creates an electric field on the ground. The voltage is that field summed along the path |
| Symbol | Meaning and unit |
| V | Voltage generated between the two ends of the wire (volts) |
| E | Strength of the ground's electric field (volts per km) |
| L | Wire length (km) |
| Ground electric field in a major geomagnetic storm | said to be a few volts per km (here, 2 volts) |
| Length of a long transmission line | 1000 km |
| Length of indoor house wiring | 30 m (0.03 km) |
| Distance from Sun to Earth | about 150,000,000 km |
| Time for the 1859 cloud to arrive | said to be about 17.6 hours |
| Voltage generated in the transmission line (V) | 2 × 1000 = 2000 |
| How many times a 100-volt home outlet | 2000 ÷ 100 = 20 |
| Voltage generated in house wiring (V) | 2 × 0.03 = 0.06 |
| 17.6 hours converted to seconds | 17.6 × 3600 = 63360 |
| Speed of the 1859 cloud (km/sec) | 150000000 ÷ 63360 ≒ 2367 |
| How many seconds for ordinary solar wind (400 km/sec) | 150000000 ÷ 400 = 375000 |
| Converted to days | 375000 ÷ 86400 ≒ 4.3 |
On the very same ground, a 1000 km transmission line generates 2000 volts, while 30 m of house wiring generates only 0.06 volts. What makes the difference isn't the storm's strength — it's the wire's length. The 1859 cloud crossed, in under a day, a distance that ordinary solar wind would take more than four days to travel.
HSHS+Why does "DC" cause trouble?
HSA transformer changes voltage by varying the magnetic flux in its iron core using alternating current and electromagnetic induction. This is covered in the high school physics units on "alternating current" and "transformers".
HS+The current generated by a geomagnetic storm changes slowly, over periods of minutes to tens of minutes, so compared with AC's 50-60 cycles per second, it's nearly direct current. When this flows in through a transformer's grounded neutral point, the core's magnetic flux gets pushed to one side. This causes the core to magnetically saturate, which is thought to lead to distorted current, heating, and extra reactive power consumption.
UnivThe ground's electric field depends on how well the Earth conducts electricity
The strength of the ground's electric field depends not just on how fast the geomagnetic field is changing, but also heavily on underground electrical conductivity. Over old bedrock that conducts electricity poorly, the field becomes stronger — and Quebec's geology is considered a classic example of this. The method of modelling the ground as flat layers to calculate this is called the "magnetotelluric method", also used in subsurface exploration. The size of a geomagnetic disturbance is expressed as the "Dst index"; in 1989 it reached about −589 nanotesla. The 1859 value is estimated, with considerable uncertainty, at somewhere between −850 and −1700 nanotesla.
📖 For the derivation of the formula and further reading: Electromagnetic Induction (Wikipedia, Japanese) ／ Geomagnetic Storm (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- How strong was the 1859 storm, really? Geomagnetic records from that era are sparse, so strength estimates vary widely. It's even debated whether carbon traces in tree rings point to even stronger solar storms further in the past.
- When will a storm of the same scale strike next? Studies estimate the probability of occurrence per decade, but the figures vary between studies.
- Can we learn the cloud's magnetic orientation before it arrives? The scale of damage depends on the cloud's magnetic orientation, but that's only knowable when it passes an observation satellite stationed ahead of Earth — about an hour before arrival.
In other words, even what's in this article is "the best explanation we have right now". Space weather forecasting is still retracing the path that ordinary weather forecasting has already walked.
Textbook connections (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | 2nd-year science, "Current and Magnetic Fields" — electromagnetic induction | A changing magnetic field generates voltage |
| HS | Physics "Electromagnetic Induction and AC"; Earth Science "The Sun and Earth" | Transformers, solar wind, and the geomagnetic field |
| HS+ | Physics (advanced): magnetic saturation | Why DC trips up transformers |
| Univ | Electromagnetism, space physics, geoelectromagnetism | Ground conductivity, the ground's electric field, Dst index |
| Research | Space weather forecasting, reconstructing past solar storms | What's still not known |
| — | Connections to daily life | Blackout preparedness, satellite positioning drift, aurora watching |
- National Institute of Information and Communications Technology (NICT, 情報通信研究機構), "Space Weather Forecast"
- Japan Meteorological Agency, Geomagnetic Observatory (気象庁 地磁気観測所)
- Solar Storm of 1859 (Wikipedia, 1859年の太陽嵐)
- Boteler, D. H. (2019). A 21st Century View of the March 1989 Magnetic Storm. Space Weather, 17.
- Tsurutani, B. T. et al. (2003). The extreme magnetic storm of 1–2 September 1859. Journal of Geophysical Research, 108(A7).
※This article is a general-audience science explainer. The figures given are approximate, meant to illustrate the underlying mechanism. In the event of a power outage or communication disruption, please follow guidance from your power company or local authorities.