The largest recorded solar storm in history, the Carrington event of 1859, may have been even rarer and more extreme than we thought, according to rediscovered magnetic data gathered at the time.
In early September 1859, a massive solar flare was seen and a coronal mass ejection 鈥� a bubble of plasma and magnetic field expelled from the sun鈥檚 corona 鈥� struck Earth鈥檚 atmosphere, triggering a powerful geomagnetic storm that produced dazzling auroras and fried telegraph wires for several days. If one of similar magnitude happened today, it could cause havoc, knocking out satellites, communication systems and power grids.
Most of our knowledge of the Carrington event comes from contemporaneous descriptions from astronomers, like Englishman Richard Carrington, or magnetic recordings taken from an observatory in India. However, neither of these accounts contain detailed numbers describing the storm鈥檚 magnetic intensity, so it has been difficult to know exactly how powerful the storm was compared with modern examples.
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Now, at the British Geological Survey and his colleagues have digitised paper recordings of Earth鈥檚 magnetic field made throughout the Carrington event taken from two observatories in London, at Kew and Greenwich. They found that the intensity and speed of change in the magnetic field during the storm indicates it was at least a 1-in-100 year event, possibly as extreme as a 1-in-1000 year event.
This brings the storm more in line with some of the original estimates of its strength in an 1861 scientific paper, which was later revised down by physicists because they thought the original recordings were inaccurate. 鈥淟ooking at the rate of change [of magnetic field intensity] just computed from the magnetograms, it鈥檚 at least 500 nanotesla per minute, which kind of supports what the original 1861 papers suggested,鈥� says Beggan. 鈥淚t just proves once again that the Carrington storm was an extreme event.鈥�
That is almost twice the expected size of a 100-year event, which would be about 350 nanotesla, he says.
To digitise the data, Beggan and his colleagues took images of the London magnetograms, which had been made using a magnetic needle suspended by a thread, the movement of which was recorded on paper to show the strength of the storm. They converted the millimetre deviations into a scale of standardised units. The needle goes off the chart for the peak of the storm, and some of the graphs are difficult to read, so there is still some uncertainty over the exact maximum strength, says Beggan.
The researchers also found readings for an apparent geomagnetic storm several days before the Carrington storm, which may have contributed to the extreme nature of the latter. This is because the previous storm may have swept away some of the solar wind 鈥� the plasma of protons and electrons flowing outwards from the sun 鈥� leaving a clearer path for the final coronal mass ejection to hit Earth more fully, says Beggan.
鈥淭his is confirmation of how extreme the event was,鈥� says at the University of Warwick, UK. 鈥淧eople talk about the Carrington event being a 1-in-100 year event, but it鈥檚 still just a bit wishy-washy. Having a published paper which quantitatively says this, is really, really valuable.鈥�
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