On Monday 22 June 2015 at about 6.25 pm GMT, NASA's Solar Dynamics Observatory witnessed a sunspot labeled AR 2371 (Active Region 2371) emit an M6.5 Solar Flare (flare emitting X-rays with an energy of more than 0.00065 Watts per meter squared). The X-ray and ultraviolet emissions from this flare caused a temporary blackout of short-wave radio signals over parts of North America and in the polar regions. The flare also caused the release of a coronal mass ejection (stream of plasma) that is traveling directly towards us and is predicted to reach the Earth on Wednesday 24 June. This is likely to produce some spectacular auroras in the polar regions, and possibly at lower latitudes, but not to cause any serious harm.
Blackout map showing areas suffering disruption to shortwave radio signals caused by the 24 June 2015 solar flare. Areas coloured red suffered the worst disruption, while areas coloured black were unaffected. Space Weather.
Sunspots are magnetic storms on the face of the Sun. These inhibit convection currents in the Sun's photosphere, causing localized cooling; the surface of the Sun can drop from its usual 5778 K to as low as 3000 K in a Sunspot, causing them to darken compared to the rest of the Sun (though they are in fact still pretty bright). Since Sunspots are magnetic they have magnetic poles, with positive and negative charges. These can be connected by coronal loops, streams of magnetic flux carrying plasma above the surface of the Sun. This can lead to a short circuit in which a large amount of magnetic energy is released suddenly, producing a brightening we perceive as a Solar Flare.
This in turn can lead to the release of a coronal mass ejection, a stream of charged particles, mainly electrons and protons but with some ionized atoms of heavier elements such as helium or oxygen. This travels out from the Sun, typically taking about two days to reach the Earth's orbit.
Solar Heliospheric Observatory movie of the 24 June 2015 coronal mass ejection. Space Weather.
When these streams of charged particles reach the Earth they can cause magnetic storms. These are usually harmless, with the energy being released in spectacular displays of light near the Earth's poles known as the Aurora Borealis (north) and Aurora Australis (south), but occasionally large events cause problems for electrical systems on Earth, such as the March 1989 event that knocked out electrical distribution networks in Quebec (such distribution networks are now generally better safeguarded against these events).
Aurora displays come in a variety of colours, caused by electrons from the coronal mass ejection striking different atoms in the Earth's atmosphere. This is because the energy of the atoms increases each time it is struck by an electron, but atoms can only absorb so much energy before they must release some, and each atom always releases energy as light (photons) at a specific wavelengths. In the Earth's atmosphere this is effected by altitude, thus Oxygen releases either green or red light and Nitrogen releases either blue or violet light. Typically auroras shimmer as different reactions occur, photographs do not really do them justice.
Aurora Australis (Southern Lights) display spotted over Bundanoon in New South Wales on 22 June 2015. This was caused by an earlier coronal mass ejection produced by a solar flare on 20 June 2015. David Metcalf/DJM Images.
Gasses release light at specific wavelengths in response to other stimuli besides coronal mass ejections. Thus the blue colour of the daytime sky is the colour of Nitrogen in the lower atmosphere reacting to the (steady) energy input from sunlight, whereas the red colour of sunrises and sunsets is the colour of oxygen higher in the atmosphere reacting to the same; we see this at dawn and dusk because the sun is no longer in line of sight with the lower atmosphere. Neon lights are red because Neon gas releases red light in response to electrical charge, and Sodium lights orange for the same reason. Molecules made up of more than one sort of atom, such as Carbon Dioxide (CO₂), Water (H₂O) or Methane (CH₄) release light in the infra-red part of the spectrum, which can lead to warming of the atmosphere (the Greenhouse Effect), hence the current concerns about the release of such gasses into the atmosphere by industrial processes, and the effect this might have on our climate.
See also...
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