An artistic illustration of Solar flare impacts on geospace. [Credit: Jing Liu]

Skies around the world glowed with dazzling aurorae. Some birds thought it was morning, some people thought it was the end of the world. The telegraph stopped working. But much spookier, sometimes telegraph operators could send messages when the power supply was disconnected. This was the Carrington Event, the biggest solar storm ever recorded.

The Sun – caught in the act
Richard Carrington, an English amateur astronomer, was observing the Sun on the morning of September 1, 1859 in his observatory in Redhill, Surrey. He had just drawn a Jupiter-sized sunspot when “two patches of intensely bright and white light broke out”. They were gone within a few minutes, but the sight was so surprising that he was relieved to find out that another astronomer, Richard Hodgson, had also seen it. Carrington and Hodgson were the first persons to see a solar flare. [This photo is from the Solar Dynamics Observatory of a flare detected in November 2020.]

At the time the flare was seen, there was a high reading on the magnetometer at Kew Observatory in Richmond, Surrey, UK. Balfour Stewart, the observatory's director, knew that there was evidence for a connection between sunspots and magnetic disturbances on Earth. So he didn't think that the timing of Carrington's observation and the magnetic reading was a coincidence. He conjectured that it was “not impossible to suppose that in this case our luminary [the Sun] was taken in the act.”

Then what happened?
Carrington saw the flare at about eleven o'clock in the morning. At five o'clock (GMT) the next morning, brilliant aurorae began around the world. Balfour Stewart noted that magnetic disturbances began at the same time.

Reports of aurorae came from both hemispheres, and from latitudes where aurorae were rare – as far south as Cuba and Hawaii in the northern hemisphere and as far north as Queensland in the southern hemisphere. A Baltimore (Maryland) newspaper reported that the aurora was brighter than the full moon. In Boston it was light enough to read a newspaper. A woman on Sullivan’s Island in South Carolina said that “the eastern sky appeared of a blood red color.” The red sky convinced some people that their city was ablaze, and a Washington DC newspaper reported that the fire department was called out.

Throughout most of human history the only noticeable effect of solar activity was the aurora. But by 1859 the telegraph was in extensive use in North America and Europe, and the network stopped working. In addition to the apparatus not working, there were instances of operators being burned or severely shocked, and of apparatus or paper catching fire. But strangest of all was that sometimes messages could be sent when the power supply was disconnected. An operator in Boston, Massachusetts and one in Portland, Maine exchanged messages for well over an hour.

What was behind the Carrington Event?
The entire global series of events caused by the solar activity of September 1-2, 1859 is what people mean by the Carrington Event. What Carrington saw and what occurred the following day are related, but not identical.

The Sun goes through activity cycles of about eleven years. Sunspots occur where intense magnetic fields reach the surface of the Sun. They show up as darker spots because they’re not quite as hot as the surrounding surface.

Carrington was observing a large sunspot when he saw a solar flare. A flare is a tremendous release of energy that's built up from magnetic activity. Solar flares release radiation. Since it travels at light speed, the bright light Carrington saw took about eight minutes to arrive from the Sun. These days a solar flare may disturb the ionosphere and cause radio blackouts at certain frequencies. However Earth's atmosphere and magnetic field shelter us from other effects.

But flares may be accompanied by coronal mass ejections (CMEs), and these have much more powerful effects. Although they travel with varying velocities, it's always well below light speed. The fast ones can take less than two days to arrive, and average ones about four days. Slower ones may take up to a week or more. They're unpredictable, so even when we know one is on the way, the arrival time can vary up to six hours either side of the forecast.

A CME is an enormous, energetic cloud of charged particles which can gain energy as it travels from the Sun. It has a strong effect on Earth's magnetic field, neutralizing some of its protection.

A good effect of a CME is the beautiful auroral displays. On the other hand, the CME's magnetic energy can interact with Earth's magnetic field to produce geomagnetic induced currents (GICs). These currents travel through the ground, which is how geomagnetic instruments detect and record them. They can also travel through power lines and pipelines – and in the case of the Carrington Event – telegraph wires. It was through geomagnetic induced currents that the telegraph was sometimes usable during the solar storm.

Our modern world is highly susceptible to a Carrington Event because of our reliance on electricity and satellites.