General Astronomy/Space Weather
Space weather is a general term that applies to the environmental conditions in space imposed largely by solar activity such as the "wind" of subatomic particles that constantly flows from the Sun. This Solar wind sometimes rages into a "storm" of charged particles (mostly electrons and protons) that can affect, disturb or even incapacitate satellites in orbit, disrupt communications, and pose radiation danger to unprotected astronauts.
The best known aspect of space weather is the aurora, commonly known as the "Northern Lights" in the Northern Hemisphere, with the corresponding "Southern Lights" in the Southern Hemisphere. These are formally known as the Aurora Borealis and Aurora Australis, respectively. they occur when large masses of the charged solar wind particles are attracted to the polar regions by Earth's magnetic field. As they spiral down through the atmosphere, they collide and interact with molecules in the atmosphere, most notably nitrogen and oxygen, which causes the emission of light seen as the aurora. The process is similar to that in an fluorescent light.
During particularly intense solar wind storms, electrical charges can collect on electrical transmission lines on Earth, sometimes causing an overload and blackouts (such as in Quebec in 1989). In addition to affecting the radio characteristics of the upper atmosphere, the solar wind charges also collect on orbiting communications satellites and can disable them unless precautions are taken. It is estimated that a solar storm of the intensity of the one in 1859 were to strike it could cause up to 2 trillion dollars damage.
Potentially more deadly are X and Gamma rays that can harm astronauts on flights well away from Earth, such as to the Moon or Mars. Space Shuttle and International Space Station astronauts are at only moderate danger since the Earth's magnetic field provides some partial protection for astronauts in low-Earth orbits (as both the space shuttle an ISS are).