At about 4.00 am GMT on Monday 23 January 2012 a massive solar flare was observed on the sun, pointing more-or-less directly our way. An hour later radiation from the flare, a stream electrons, followed by a wave of protons (hydrogen ions) moving at 41.6 million meters per second washed over the Earth, a stream that we will remain in till Wednesday 25 January. This is a long way from being the worst solar storm ever recorded, but is the worst for several years, and may cause problems for satellite communications systems and astronauts on the International Space Station. On the bright side, it may lead to some spectacular Aurora Borealis displays, which may come south further than usual (the flare is angled slightly to the north of the Earth, so increased Aurora Australis displays are unlikely).
Space Weather prediction animation from the US National Atmospheric and Oceanic Administration.
Solar flares occur when energy builds up in the Sun's magnetic field, then is transfered to charged particles in the form of kinetic energy, though exactly how this happens remains a mystery. The result is a wave of charged particles that hits the Earth, these charged particles then interact with atoms in our upper atmosphere, causing a release of photons (light), which we see as auroras. Nitrogen atoms emit green or blue light when they interact with charged particles, oxygen atoms green or red light; thus green is the most common colour in aurora displays on Earth. The displays tend to happen at the poles as the Earth's magnetic field channels the charged particles their; the Earth is effectively a huge bar magnet and the charged particles act like iron filings, seeking out the poles. In times of high solar activity the number of charged particles goes up, and the auroras spread away from the poles.
The Aurora Borealis seen from Ineshowen in County Donegal on Sunday 22 January 2012 (before the solar flare). Image from Adam Porter of the Buncrana Camera Club.
In extreme instances solar flares can interfere with electrical and communications networks. In 1989 a solar storm caused a blackout in Quebec that lasted 9 hours. The rocks of the Canadian Shield (on which Quebec sits) are poor conductors of electricity, so a build up of static electricity caused by the storm tried to earth itself through the power grid, burning out sections of the network and tripping circuit breakers across the state. Since then Hydro-Québec have introduced special measures to deal with solar storms, as have some other power networks in North America and Europe. The storm also caused problems for communications satellites and the Space Shuttle Discovery, which was in orbit at the time. On that occasion the Aurora Borealis was seen as far south as Texas.
See also Comet seen falling into the sun.