On 1 September 1859, Richard Carrington had just completed his morning observation of the sun when he saw a pair of bright crescent shapes forming near a large group of sunspots. For years, Carrington had been observing the sun by projecting its image onto a screen and meticulously sketching what he saw. He had never seen anything like this before. Excitedly, he dashed through his observatory trying to find someone to witness his discovery. But the building was deserted, and when he hurried back to his telescope he was horrified to find that the vision was already fading.
Carrington had seen a solar flare, making him the first person as far as we know to witness such an event. But his flare was not only the first on record, it was also spectacularly large. Today, new evidence suggests that Carrington had witnessed the onset of what would prove to be the perfect solar storm.
DAY after day, year after year, Richard Carrington mapped sunspots. Wealthy enough to build himself a house near London with an observatory attached, Carrington had set out to follow the movement of sunspots throughout the sunspot cycle. In the process he had discovered that as the cycle progresses, the spots migrate towards the equator. He also discovered that the sun does not rotate as a solid object, but at different rates at the poles and the equator. Then, on 1 September 1859 he was counting spots 鈥渨hen within the area of the great north group, two patches of intensely bright and white light broke out鈥�.
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As Carrington reported to the Royal Astronomical Society: 鈥淪eeing the outbursts to be very rapidly on the increase, and being somewhat flurried by the surprise, I hastily ran to call someone to witness the exhibition with me, and on returning within 60 seconds, was mortified to find that it was already much changed and enfeebled. Very shortly afterwards the last trace was gone.鈥�
Luckily another English astronomer called Richard Hodgson also saw the flare and reported his observations. History, however, has given Carrington the credit for the discovery. The bright crescents were eruptions from a massive solar flare now known as the 鈥淐arrington event鈥�.
At the time, the Carrington event was an interesting celestial phenomenon. Today it is causing considerable consternation. The reason? Carrington鈥檚 solar outburst was a super-giant among space storms, surpassing even the monster eruption of March 1989, which knocked out the entire power grid in Quebec, and the huge 鈥淗alloween storms鈥� late last year, which disabled two satellites and disrupted communications and power grids around the world. A solar storm the size of Carrington鈥檚 would pose a huge risk to satellites and spacecraft. On Earth, it could knock out so many big transformers that it would take weeks, perhaps even months, to restore power, warns John Kappenman, of Metatech, a company in California that advises power companies how to protect against solar storms.
Even at the time, Carrington and his contemporaries realised that something odd had happened. Within minutes of Carrington鈥檚 first observation of the bright white light, magnetometers at the observatories at Greenwich and Kew gave a large jerk as something radically altered the Earth鈥檚 magnetic field. Then 17 1/2 hours later, the magnetometers jumped off the scale. At about the same time, telegraph operators reported sparks leaping from their equipment, some bad enough to start fires and melt wires. And the night skies were filled with colour as the northern lights strayed as far south as Cuba and Honolulu.
Carrington almost certainly suspected that these events were linked to his flare, but scientists of his era were cautious types, not readily given to jumping to conclusions. 鈥淥ne swallow does not make a summer,鈥� he later noted. We now know that the first magnetic jolt heralded the arrival of a shock wave of highly energetic particles, moving at nearly the speed of light. Subsequent sparks, fires and psychedelic skies were triggered by slower-moving particles that carried most of the storm鈥檚 energy.
The speed at which the later surge of particles arrives is a marker of the energy with which they are blasted out of the sun. After a typical flare, the delay is about 30 hours. At 17 1/2 hours, the Carrington event is the second fastest on record. Top honours go to a storm in August 1972, which reached Earth in 14 1/2 hours. Last year鈥檚 storms took 19 hours to hit Earth.
That alone meant that the Carrington event was large. But just how large? At the joint assembly of the American and Canadian geophysical unions in May this year, researchers following several different lines of enquiry produced an answer: by some measures it was two to four times as large as the storm in March 1989 that left Quebec in the dark and which had previously been the benchmark against which all solar storms were measured.
One startling aspect of the Carrington event was the extremely low latitudes at which auroras were observed. In a paper published shortly before the meeting, Bruce Tsurutani at the Jet Propulsion Laboratory in California noted that red glows were visible as little as 23 degrees from the geomagnetic equator 鈥� a record that still stands. He also reported that while the English magnetometers jumped off the scale and provided no meaningful data during the main portion of the storm, a forgotten instrument near the Indian city of Bombay continued to give readings. Recent calculations by researchers at the Indian Institute of Geomagnetism reveal that the storm was brief but intense, providing a valuable clue to the mechanism that produced it. And while modern researchers are reluctant to rely too heavily on the precise readings from the Bombay magnetometer, Tsurutani is confident that they reveal the most intense magnetic storm on record.
More importantly, Tsurutani knew that the lowest latitude to which auroras extend is an indicator of the strength of a magnetic storm. He also now knew the type of storm, as well as the time it took the Carrington event to hit Earth. Feeding this data into a model based on observations of more recent solar storms, he calculated that at its peak, the storm had caused the Earth鈥檚 magnetic field to fluctuate by 1760 nanotesla (nT). By comparison, the largest such fluctuation ever recorded by modern instruments was 590 nT, during the 1989 solar storm. Tsurutani鈥檚 calculation indicates that the 1859 magnetic storm was not just the most intense in recorded history, it was three times as intense as the one in 1989.
Support for this assertion comes from a very different source: polar ice cores. Peggy Ann Shea and Donald Smart of the US Air Force Research Laboratory in Bedford, Massachusetts, examined an ice core from Greenland that spans the period from 1561 to 1992, looking for traces of gigantic solar flares. Their aim was to measure the flux of particles carrying energy greater than 30 million electronvolts (MeV). These are the super-energetic particles that pose the greatest risk to satellites and spacecraft. They are also the ones that cause the magnetic disturbances, light up the night skies with auroras and create electrical surges that wreck power equipment on Earth.
When these particles hit the upper atmosphere, they trigger a cascade of chemical reactions that produce a dramatic spike in the amount of nitrates settling onto the polar snows, a signal preserved when the snow is compressed into ice. The Carrington event produced by far the largest spike in the core, producing about four times as many high-energy protons as the August 1972 event, which had previously topped the league by this measure.
Rating solar storms is difficult because there are at least four scales by which they can be measured: the 30 MeV flux; the latitudes at which auroras can be seen; the size of the magnetic fluctuations; and ground disturbances such as power blackouts or, in this case, fires. The Carrington event may not have topped all of these scales, but it is exceptional in every category. 鈥淚 can鈥檛 think of any other event for which I can say that,鈥� says space physicist Ed Cliver of the USAF Research Laboratory.
It is a combination that adds up to the ultimate solar storm. Could it happen again? 鈥淵es,鈥� Tsurutani says without hesitation. How soon? Nobody knows.