At midnight on Tuesday 6/Wednesday 7 March 2012, GMT, the Sun emitted the largest solar flare since 9 August 2011; the second largest since its current last solar minimum in 2007 (solar activity increases and drops in an eleven years cycle). The flare was classified as an X4 type flare, the most severe form of flare, which translated means that during the flare the sun was emitting more than 0.001 Watts per meter squared (W/m²) over more than o.06% of its surface area.
This was followed at 1.14 am GMT by a second flare from the same part of the sun, which registered as an X1.3 class flare (emitting more than 0.001 W/m² over more than 0.01% of the Earth's surface), according to NASA's Solar Terrestrial Relations Observatory and Solar Heliospheric Observatory.
Short video showing the solar flare at two different wavelengths. NASA/Goddard Space Flight Center.
Both these flares are likely to hit the Earth (and Mars, which is currently in alignment with Earth). The larger flare is traveling at slightly over 2000 km/s², and is expected to reach the Earth on 8 March at about 6.25 pm, the second at slightly under 1800 km/s². These will be followed by two (related) coronal mass ejections, moving towards the Earth at slightly under 1000 km/s².
The flares consist of clouds of electrons, atoms and charged ions flooding outwards from the sun (generally electrons first). These can interact with particles in the ionosphere to cause auroras (Borealis in the northern hemisphere, Australis in the south). This happens as atoms or molecules in the upper atmosphere gain energy from the flare. An atom or molecule can only gain so much energy above its ground state then it has to release some. This happens in bursts of energy at specific wave-lengths specific to each atom and/or molecule. Nitrogen (N₂) and oxygen (O₂) both emit energy in the blue part of the spectrum, giving us a blue sky. Water (H₂O), methane (CH₄) and carbon dioxide (CO₂) all emit light in the infra-red part of the spectrum, which is why too much of these gasses in the atmosphere can cause global warming. Hydrogen (H₂) and helium (He) emit light in the red and green parts of the spectrum, respectively giving us the colours of the Northern and Southern Lights.
The Aurora Australis seen from above. A stream of charged helium ions (He+) from a coronal mass ejection interacting with the Earth's ionosphere in May 2010, as seen from the International Space Station. NASA/Earth Observatory.
There is also a danger that solar flares might interfere with energy distribution networks, though these are increasingly protected against such effects, and/or damage satellites. As well as numerous commercial satellites, some high profile NASA missions could potentially be effected by these flares, including Messenger, Spitzer, and STEREO-B.