On the morning of September 1, 1859, Richard Carrington, an English amateur astronomer, was sketching sunspots at his private observatory at Redhill, south of London. He was projecting the Sun's image onto a screen, which is the safe way to watch it. Without warning, two patches of intensely bright white light appeared inside a large group of sunspots. They brightened, shifted position, and faded within about five minutes.
Carrington knew he was looking at something unusual. He hurried off to find a witness, and by the time he returned the light was already fading. Another observer, Richard Hodgson, independently reported seeing the same flash.
It was the first recorded observation of a solar flare. What followed over the next day and a half came to be called the Carrington Event, and it is still the best-known space weather storm in history.
What arrived at Earth
Within about 17 to 18 hours, an unusually fast cloud of charged particles from the Sun reached Earth. Today this is called a coronal mass ejection. When it struck Earth's magnetic field, it set off a severe geomagnetic storm.
The most visible effect was the aurora. On the nights of August 28 to 29 and September 1 to 2, 1859, two storms struck in quick succession, and the second was enormous. Auroras were reported far from the poles, including in the Caribbean, Hawaii, Mexico and parts of South America. In the northeastern United States, accounts say the sky was bright enough to read a newspaper by. Some people in the Rocky Mountains reportedly woke up, took the glow for dawn, and started making breakfast.
The telegraph problem
In 1859 the most advanced communication network in the world was the electric telegraph. It ran on long wires strung across whole countries and, by then, under some seas.
Geomagnetic storms induce electric currents in long conductors. During the Carrington storm, those currents threw telegraph systems across Europe and North America into disorder. Operators reported sparks jumping from their equipment. Some said they had received shocks. In some machines the paper strips were scorched.
The best-known detail is that operators on a line between Boston and Portland, Maine, disconnected their batteries and found they could still send messages, running on nothing but the current induced by the storm. That account comes from reports published in newspapers and scientific journals of the day, and historians of space weather cite it widely.
How we know it was so strong
There were no satellites in 1859, so scientists have to work from indirect evidence.
- Magnetometer records. The observatory at Colaba in Bombay (now Mumbai) recorded a very large drop in the horizontal component of Earth's magnetic field. Estimates drawn from that record are often used to rank the storm against others.
- Aurora reports. The low latitudes at which the aurora was seen are a strong sign of an extreme storm.
- Transit time. The short gap between the flare and the storm points to a very fast coronal mass ejection.
Researchers once suggested that ice cores held a nitrate spike from the 1859 event. That claim was later questioned, and more recent studies concluded that the nitrate signal is not a reliable record of solar storms. It is a useful reminder that a detail repeated widely can still be revised.
How rare is a storm like this?
The Carrington Event is the largest geomagnetic storm in the instrumental record, though others have come close.
- May 1921. A severe storm disrupted telegraph and telephone service and was linked to fires at some telegraph and railway signaling facilities.
- March 1989. A storm caused the Hydro-Québec power grid to collapse in about 90 seconds, leaving millions of people in Quebec without power for around nine hours.
- July 2012. A very large coronal mass ejection erupted from the Sun but was not aimed at Earth. NASA scientists later said that had it happened about a week earlier, it would have hit us. That near miss comes up often in discussions of risk.
Estimates of how often a Carrington-level storm occurs vary, from about once a century to less often than that. The honest answer is that the record is not yet long enough to be sure.
Why it matters more now
In 1859 a severe storm made telegraphs spark and gave people a spectacular night sky. A storm of the same size today would hit a world that runs on electricity and satellites.
- Power grids can be damaged when induced currents overload large transformers, and some of those transformers take months to replace.
- Satellites can suffer electronic damage, as well as extra drag from an upper atmosphere that swells during a storm. In February 2022, SpaceX lost dozens of newly launched Starlink satellites after a relatively minor geomagnetic storm increased atmospheric drag.
- GPS and radio communications can be degraded.
Governments now treat space weather as a serious hazard. In the United States, the NOAA Space Weather Prediction Center issues forecasts and alerts using a scale that runs from G1 to G5 for geomagnetic storms. The United Kingdom lists severe space weather on its National Risk Register.
A closer recent example
On May 10 to 11, 2024, a G5 storm, the top level on NOAA's scale and the first since 2003, produced auroras across much of the United States and Europe, far south of where they normally appear. Power systems came through without major failures, in part because operators had warning and took precautions. It was not a Carrington-level storm, but it gave millions of people a glimpse of what Carrington's contemporaries saw.
What the record supports
- Carrington's observation is well documented in his own report to the Royal Astronomical Society.
- The aurora sightings are documented in many newspaper and scientific reports.
- Telegraph disruptions are well documented. Some specific anecdotes, such as individual operators' shocks, rest on single reports.
- The storm's strength is estimated from limited data and is still debated in the scientific literature.
Sources
- R. C. Carrington, "Description of a Singular Appearance seen in the Sun on September 1, 1859," Monthly Notices of the Royal Astronomical Society, vol. 20 (1859).
- NASA, "A Super Solar Flare" (2008) and NASA Science materials on the July 2012 near miss.
- NOAA Space Weather Prediction Center, geomagnetic storm scales and May 2024 storm summaries.
- E. W. Cliver and L. Svalgaard, "The 1859 Solar-Terrestrial Disturbance and the Current Limits of Extreme Space Weather Activity," Solar Physics, 2004.
- Hydro-Québec and North American Electric Reliability Corporation reports on the March 1989 blackout.