One of the biggest geomagnetic storms on record was the Carrington event of 1859.

BBC: System could warn of solar storms

When a massive solar flare erupts on the sun, it can cause geomagnetic storms on the Earth.

CNN: Four ways the Internet could go down

The sun has spewed out numerous intense spikes of energy over the past week, including several that did hit Earth, producing geomagnetic storms and some of the best aurora displays in years.

CNN: Sun lets loose monumental flare