The sun is a source of energy, and gives off huge amounts of light and radiation. But the sun also gives off a constant stream of charged particles ... mostly protons and electrons ... that flow outwards from the sun at speeds of about 400 km/s (about 1 million miles per hour).
This 'solar wind' is enhanced by even more high-speed particles given off by solar flare sand sunspots. In the close-up at the right, a flare is seen looping above the right side of the sun; sunspots are the slightly cooler, darker areas on the surface, which indicate turbulence. Fast-moving atomic particles are given off by sunspots, and will cause auroras when they reach the earth.
The earth has a magnetic field, and the lines of magnetic force emanating from the earth trap the particles from the sun, and pull them downwards. Much like a bar magnet, the lines of force concentrate at the north and south poles ... and that's where most of the energetic particles from the sun end up.
As they spiral down through the atmosphere near the poles, the particles collide with molecules of air, especially oxygen. The oxygen molecules get a boost of energy, which they immediately give off again. This energy being released by 'excited' oxygen molecules is visible as a green glow ... which we see as a shimmering curtain of light.
When an oxygen atom is hit by a solar particle, an electron is knocked out, ionizing (or 'exciting') the oxygen atom. When a free electron is later reacquired by the oxygen atom, it emits radiation of a blue-green wavelength. All this takes place in the high atmosphere ... auroras happen at heights of 100 kilometres and higher.
Because the increase in solar wind from sunspot activity on the sun follows the same pattern as sunspot growth, the auroras we see on earth peak in intensity every 11 years just as sunspots do. As you can see from the graph on the right, sunspot activity peaked in 2001, as did the size and frequency of auroras.
Both are now starting to decline, following an 11 year cycle. The reason for this 11-year cycle in sunspot activity has to do with the inner dynamics of the sun, and is as yet not well understood. Nevertheless, the cycle is real, and keeping watch on sunspot or flare activity is important.
High energy particles from the sun greatly increase in number when there are flares on the sun, or a lot of big sunspots. This increased activity is called a solar 'storm'. Not only do these particles excite oxygen atoms in our atmosphere and make them glow, but they also can be harmful, both to equipment and people.
Most of the particles are scooped up by the earth's magnetic field, or absorbed by the atmosphere, so they don't make it to the surface. However, astronauts could be harmed by this increase in radiation, so activities are scheduled around major solar activity. Unmanned satellites, however, are in space for years at a time, and the electronics in these are sometimes damaged during solar 'storms'. Radio communication can be disrupted. Even people aboard high -flying airplanes can be subjected to a dose of radiation equivalent to about one x-ray.
