The largest solar storm in history, the Carrington event, was even bigger than we realized

Last night Earth experienced its strongest geomagnetic storm since 2017, but it was small compared to the storm caused by the most powerful solar storm ever recorded in 1859, known as the Carrington Event. Now we learn that the disruption to Earth’s magnetic field during the Carrington Event was even greater than previously estimated. A combination of modern digital tracking and detailed reconstruction has collected data about Earth’s magnetic field at the time and revealed more than thought possible. The finding reinforces how vulnerable modern society could be to a recurrence of an event like this.

On September 1, 1859, the sun spewed electrified gas and subatomic particles with the energy of 10 billion atomic bombs at the planet, blacking out telegraph communications, literally shocking operators and setting systems on fire. Northern Lights were reported as far south as Cuba and Hawaii, leaving witnesses to read newspapers only in the light of the aurorae.

Solar storms have occurred throughout Earth’s existence. However, our scale estimates were based on very indirect measurements, such as the presence of certain radioisotopes in tree rings. In recorded history, reports of huge auroras can indicate the timing of solar storms, but are of little use in estimating their size. As a result, our data on how big solar storms can get goes back less than two centuries.

Coincidentally, the Carrington Event, by far the largest storm at the time, occurred when such tracking was still in its infancy. It has been revealed that the data created at the time contained more information about the Event than was thought, and that is not good news for those preparing for future consequences in a more wired world.

If the Carrington Event had occurred even a few decades later than the actual date of 1859, there would have been electricity and long railway lines to electrify, not just telegraphs. But at least we would have known the extent of it better.

Nevertheless, both the British Greenwich and Kew observatories had magnetograms which measured fluctuations in the strength and direction of Earth’s magnetic field, later shown to be primarily a response to solar activity.

Since 1838, local geomagnetism had been measured in Greenwich by shining light on mirrors at the ends of magnetized pieces of metal suspended so as to swing freely, with the reflected light falling on light-sensitive paper. Kew joined two years before the big storm.

As solar activity disrupted the Earth’s magnetic field, the magnets rotated, causing light to move across the paper. The stronger the disturbance, the further the light shifted. The paper was mounted on a slowly rotating drum, similar to the disaster movies we have been taught to associate with seismometers.

Unfortunately, neither system was built in anticipation of the geomagnetic field taking a hit about as strong as 1859 had in store. As a result, the mirror-bearing metal swung so wide that the beam of light disappeared from the photographic paper for twelve hours during a magnetic storm preceding the Carrington event, and again during the event itself. Such large movements tell us that these were two extremely strong incidents, but not how strong.

It is here that digitizing the magnetogram data has proven to be an unexpected boon. The paper documents have been carefully archived and, according to a team led by Dr Ciaran Beggan of the British Geological Survey, ‘are in relatively good condition considering their age and method of preservation.’ After being carefully removed from their bindings, the daily documents were photographed and digitized, creating a continuous series rather than disconnected days.

By measuring the speed of movement of the light beams before they left the paper and after they returned, the authors calculated the rate at which the field changed, which they estimated at a minimum of 500 nT/minute. Considering that once-a-century storms are expected to cause changes of 350-400 nT/min at the London latitude, even the bottom value is extraordinary.

Besides the problem of how far the light swung from the paper, it is not easy to translate the measured movements into modern SI units. Nevertheless, Beggan and co-authors performed detailed reconstructions using comparisons between the two measurements to translate the movements into nanotesla changes in field strength. Changes in the orientation of the field are at least as important.

Two years after the Carrington Event, a scientific paper based on this type of data assessed its power and came to similar conclusions. However, 20th century astronomers, who had never experienced anything this large, concluded that the original estimates must have overestimated this.

“Looking at the rate of change… it is at least 500 nanotesla per minute, which goes some way to supporting what the original papers from 1861 suggested,” Beggan told New Scientist. “It just proves once again that the Carrington storm was an extreme event.”

Competing scientific societies developed these magnetograms because, before GPS, the Earth’s magnetic field was crucial to navigation. As early as the 17th century, Edmond Halley led voyages to map the way the field was changing across the Atlantic Ocean, before realizing that changes over time also had to be taken into account. It’s a shame that the two sets of records we have were only 20 kilometers apart, hardly representing global coverage, but more fragmentary data was collected from Finland, India and Guatemala, among others.

The research is open access in Space Weather.

Leave a Reply

Your email address will not be published. Required fields are marked *