by Lee T. Macdonald
A solar explosion that took place in 1859, known today as the ‘Carrington Event’, is used as a benchmark for a catastrophic ‘space weather’ incident that could have serious consequences for today’s mobile phone, internet and satellite communications and also for the world’s electricity supplies. Space weather events are one of the potential catastrophes on the British government’s National Risk Register of Civil Emergencies. Funding is currently being sought for a British space-based observatory called Carrington-L5, whose purpose is to continuously monitor the Sun’s emissions and provide five-day warnings of solar storms that could damage terrestrial communications.
Who was Carrington? Also, what exactly happened in the ‘Carrington event’ of 1859?
For some years around the mid-nineteenth century, Richard Christopher Carrington (1826-1875) was one of Britain’s leading astronomers. Born into a wealthy brewing family, Carrington studied mathematics at the University of Cambridge, where he graduated in 1847. Attaining high marks in the Cambridge ‘Mathematical Tripos’ exam was in those days the standard qualification for anyone who aspired to one of the relatively few paid posts in British astronomy.
After graduation, Carrington worked for several years at the Durham University observatory. In 1852 he resigned and moved to Redhill, Surrey, where he worked as an independent astronomer, setting up a private observatory with the profits from his father’s brewing firm. Carrington initially became famous for compiling a catalogue of stars near the north celestial pole, which won him the Gold Medal of the Royal Astronomical Society.
Not long after moving to Redhill, Carrington also began systematically observing the Sun and its ever-changing dark spots. Little was then known about what sunspots were. But since the 1830s, scientists had been seriously studying the Earth’s magnetic field. Then, in 1843, the German astronomer Heinrich Schwabe discovered that the number of sunspots visible rose and fell in a cycle of about ten years. Seven years later, Edward Sabine, a British army officer who was in charge of Britain’s geomagnetic observations, found a ten-year cycle in the frequency and intensity of magnetic variations. He then noticed that this cycle coincided exactly with Schwabe’s sunspot cycle. This seemed to confirm the existence of some kind of link between sunspots and terrestrial magnetism, and led to an increased interest among astronomers in observing the Sun.
It was against this background that Carrington began regularly observing the Sun. He used a 4 ½-inch equatorially-mounted refracting telescope to project an image of the Sun onto a large sheet of glass painted white, from which he made drawings of the sunspots and measured their positions. After several years of careful observation, Carrington discovered what later came to be known as ‘Spörer’s Law’: a gradual decline in the average solar latitude of sunspots as the solar cycle progressed. He also discovered that sunspots close to the solar equator rotate around the Sun faster than those at higher latitudes. In addition, he established a system of reckoning the Sun’s rotation that is still used today.
On the morning of 1 September 1859, Carrington was making his usual daily observation of the Sun. For some days past, a large and unusually complex sunspot group had dominated the solar disc. According to his own account, at 11:18am on 1 September, Carrington noticed two dots of intensely bright light appear in the big sunspot group. These bright dots gradually faded, and disappeared altogether five minutes after Carrington first noticed them. But while they remained visible, Carrington observed them move across a large part of the sunspot group – around 35,000 miles, according to his measurements.
By good luck, another British astronomer, Richard Hodgson, was also observing the Sun at the same time that morning, and independently witnessed the same phenomenon. Both presented their results at the November 1859 meeting of the Royal Astronomical Society. By then, Carrington had learned something more exciting. One or two days after 1 September, Carrington visited King George III’s former observatory in Richmond, which was by then known as Kew Observatory and which Sabine had begun using as a principal station for his geomagnetic work. Eighteen months earlier, Sabine and others had established at Kew a programme of regular photographs of the Sun’s disc, in combination with a continuous record of the terrestrial magnetic field using a set of self-recording magnetometers.
As it happened, Kew Observatory had not obtained any pictures of the Sun on the day of Carrington and Hodgson’s phenomenon. However, when Carrington and an assistant at the observatory, Charles Chambers, examined the magnetometer data, they found a pronounced jump in the traces produced by the magnetometers at 11:20am on 1 September, two minutes after Carrington had first noticed the bright spots. Moreover, on the night of 2 September, a great display of the aurora was visible over much of the globe. In the northern hemisphere, it was seen as far south as Cuba. The magnetometers at Kew and elsewhere recorded wild variations. Over the same period, operators using the recently-invented electric telegraph had problems with sending and receiving messages. The event came to be known as a ‘magnetic storm’.
Ever the careful scientist, Carrington noted the coincidence in time between the magnetic disturbances and his observation of the strange phenomenon on the Sun, but expressed diffidence about making any connections between the two, quoting the old saying, ‘one swallow does not make a summer’.
What Carrington saw on 1 September 1859 is now called a ‘white-light flare’. A solar flare occurs in an active region whose magnetic field becomes so twisted and complex that it effectively ‘short-circuits’, causing a huge release of energy. Most flares require special filters to be seen from Earth, but a few are so intense that they can be seen in ordinary, ‘white’ light. Carrington and Hodgson were lucky enough to observe an exceptionally powerful flare many years before special equipment was invented for observing these phenomena.
The ‘jump’ recorded by the magnetometers at the same time as Carrington’s flare is now recognised to have been a ‘geomagnetic crochet’, caused by ultraviolet rays from the flare ionizing the Earth’s upper atmosphere and thus exciting the terrestrial magnetic field. The longer-term magnetic variations and aurorae were caused by a great wave of subatomic particles released from the Sun by the flare, now known as a ‘coronal mass ejection’ (CME). Yet the aurorae did not occur until the evening of 2 September, about eighteen hours after Carrington’s flare, because that is how long it took the particles to travel from the Sun to the Earth. The geomagnetic crotchet occurred at the same time as Carrington saw the flare, because both the visible light seen by Carrington and the ultra-violet rays that caused the magnetic jump were just different forms of electromagnetic radiation, travelling at the speed of light.
The Carrington Event had great contemporary importance in Victorian science. It heightened an already increasing interest in Sun-Earth connections, and helped stimulate astronomers to look for further connections, including possible solar influences on terrestrial weather that might be used to predict droughts and associated famines.
However, the event’s significance for the twenty-first century is that it was one of the most powerful solar explosions ever recorded. The largest flare of modern times occurred on 4 November 2003. This originated in a complex sunspot group similar to the one that caused the Carrington flare. Across much of its passage across the Sun the previous two weeks, the 2003 sunspot had been unleashing many flares and CMEs. These had not only sparked powerful aurorae: the magnetic effects caused damage to communications satellites and some airlines flying near the arctic regions had to be re-routed, due to dangerous radiation levels in the upper atmosphere. By 4 November, when the most powerful flare took place, the parent sunspot was moving off the Sun’s visible disc and the resulting CME was directed at 90 degrees to the Earth. Had it travelled directly towards the Earth, its consequences for communications systems and transport could have been devastating.
Research by scientists into the recorded magnetic effects of the 1859 Carrington flare suggests that it might well have been as powerful as the 2003 one. The explosion’s effects on the Victorian electric telegraph were as nothing to the consequences of a Carrington-type event for the communications and power supplies we rely on in the modern world. That is why the Carrington event forms a benchmark for a potentially disastrous modern-day space weather event – and why scientists and governments need to understand and monitor the Sun’s emissions, in preparation for another such event.