Geomagnetic storms are brief disturbances in Earth's magnetic field and atmosphere (aka the magnetosphere) caused by bursts of radiation and charged particles emitted from the Sun.
When this solar matter collides with our planet at high speeds, the surrounding magnetic field deflects it towards the poles. There it interacts with gases deeper in the atmosphere to emit 'curtains' of light known as auroras.
Meanwhile, the fast-moving charges create an intense magnetic field of their own, inducing another set of electrical currents on the ground far below.
Under strong solar activity, those currents can be strong enough to overpower whole electrical grids and destroy sensitive electronics. Recent declassified naval documents suggest in 1972, geomagnetic storms even triggered the detonation of dozens of sea mines off the Vietnam coast.
One of the most severe storms, dubbed the Carrington Event, occurred in 1859 and was not so easily overlooked. It was marked by an intense brightening of auroras and reports of telegraph systems malfunctioning, electrocuting operators.
Today, there are concerns that a similar event would have far worse implications for technology and for the societies who rely heavily upon it.
What causes a geomagnetic storm?
The Carrington Event was named after the British astronomer Richard Christopher Carrington, who witnessed the beginnings of a storm on 1 September 1859.
While observing a cluster of particularly large sunspots using his solar telescope, Carrington noticed two dots within the dark patches brighten and fade over a number of minutes. Days later, the astronomer and his assistant discovered magnetometer data at what was Kew Observatory in Richmond showed a spike of activity.
Carrington didn't immediately think the two were connected. "One swallow doesn't make it summer," he famously said. Today we know better.
The brightening he saw was a white light flare. They're caused by reconfigurations in the Sun's magnetic fields, releasing huge amounts of plasma and radiation in stages.
First, there is a flash of electromagnetic radiation, which takes barely a few minutes to strike Earth. Next, protons and electrons are accelerated close to the speed of light in jets of plasma, lasting anywhere from a few hours to a day or two.
During intense periods of solar activity, 'twists' and turns in the Sun's magnetic field snap and reconnect. This can expel a slow-moving cloud of charged particles in a final stage of solar storm known as a coronal mass ejection.