Natural Phenomena: Aurora

Aurora (Latin word aurora, "sunrise" or the Roman goddess of dawn), is a natural light display in the sky. predominantly seen in the high latitude (Arctic and Antarctic) regions. Auroras are caused by charged particles, mainly electrons and protons, entering the atmosphere from above causing ionisation and excitation of atmospheric constituents, and consequent optical emissions. Incident protons can also produce emissions as hydrogen atoms after gaining an electron from the atmosphere.

The dancing lights of the aurora provide spectacular views on the ground, but also capture the imagination of scientists who study incoming energy and particles from the sun. Aurora are one effect of such energetic particles, which can speed out from the sun both in a steady stream called the solar wind and due to giant eruptions known as coronal mass ejections or CMEs. After a trip toward Earth that can last two to three days, the solar particles and magnetic fields cause the release of particles already trapped near Earth, which in turn trigger reactions in the upper atmosphere in which oxygen and nitrogen molecules release photons of light. The result: the Northern and Southern lights.
NASA's suite of heliophysics spacecraft track how events on the sun affect near-Earth space, including several missions dedicated to aurora studies. Auroras are but one symptom of a larger space weather system in which solar material and radiation can affect Earth's own magnetic environment and block radio communications, disturb onboard satellite computers, or -- at their worst -- cause electrical surges in power grids.

• Red: At the highest altitudes, excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms and lower sensitivity of eyes at this wavelength make this colour visible only under more intense solar activity. The low amount of oxygen atoms and their gradually diminishing concentration is responsible for the faint appearance of the top parts of the "curtains".
• Green: At lower altitudes the more frequent collisions suppress this mode and the 557.7 nm emission (green) dominates; fairly high concentration of atomic oxygen and higher eye sensitivity in green make green auroras the most common. The excited molecular nitrogen (atomic nitrogen being rare due to high stability of the N₂ molecule) plays its role here as well, as it can transfer energy by collision to an oxygen atom, which then radiates it away at the green wavelength. (Red and green can also mix together to produce pink or yellow hues.) The rapid decrease of concentration of atomic oxygen below about 100 km is responsible for the abrupt-looking end of the lower edges of the curtains. 
• Yellow and pink are a mix of red and green or blue.
• Blue: At yet lower altitudes atomic oxygen is, uncommon, and ionized molecular nitrogen takes over in producing visible light emission; it radiates at a large number of wavelengths in both red and blue parts of the spectrum, with 428 nm (blue) being dominant. Blue and purple emissions, typically at the lower edges of the "curtains", show up at the highest levels of solar activity.
  
  
  
  
  
  
  
  
  
  lower edges of the "curtains", show up at the highest levels of solar activity.