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Where do auroras come from?
The sun spews out charged particles traveling at a million miles per hour known as the solar wind. Particles which stream out of sunspots may be captured by the Earth's magnetic field and create the aurora. The bombardment of the solar wind would be deadly to life on Earth. The currents themselves cannot be seen, but they power the colorful, flickering nighttime display of the aurora borealis -- what in the Northern Hemisphere is known as the Northern Lights -- and the aurora australis in the south.
Earth's magnetic field deflects the streams of charged particles from the sun -- electric currents, in essence -- and either deflects them around the planet or channels them toward the North and South Poles
Sunspots are patches created by strong magnetic fields on the surface of the sun. Because these areas are somewhat cool compared to their surroundings, they look dark. Sunspots are the visual indication of the process that sends charged particles into space.
Saturn's ultraviolet aurora viewed by Hubble Space Telescope. Unlike the Earth, Saturn's aurora is only seen in ultraviolet light. Ripples and overall patterns that evolve slowly, independent of planet rotation. The pink aurora features are dominated by atomic hydrogen, with some molecular hydrogen emissions.
Charged particles, mostly protons and electrons are constantly emitted from the outer region of the Sun (corona) and reach the magnetosphere of Earth having a velocity of some 350 to 400 kilometers per second. Magnetosphere is the space around the Earth pervaded by its magnetic field. The charged particles flow around the comet-shaped magneotesphere and form eddies and distortions behind the obstacle. Electrons and protons collide with the gas particles in the outer atmosphere (altitude between 100 to 500 km). A small part of the collision energy is used for excitation of electrons in the gas particles -- mainly nitrogen molecules and oxygen atoms. The excited electrons return to their ground states by emitting light of distinct wavelength and thus a distinct color. The wavelength of the light depends on the electronic structure of the particle itself and on the energy of the charged particle colliding with the atom or molecule. Oxygen atoms emit green light at high collision energies and red light at lower energies, whereas nitrogen molecules emit blue and violet light. Green polar light can be observed in the altitudes around 100 to 200 km as only particles with high energy can penetrate into the lower layers of the atmosphere, the red light is formed in higher altitudes of 200 to 500 km due to lower energies of the colliding electrons.
The particles which stream down the magnetic field of the Earth, reach the neutral atmosphere in a rough circle called the auroral oval. This circle, or annulus, is centered over the magnetic pole and is around 3000 km in diameter during quiet times. The annulus grows larger when the magnetosphere is disturbed. The location of the auroral oval is generally found between 60 and 70 degrees north and south latitude.
Color of excited atoms
The aurora is caused by the interaction of high energy particles (usually electrons) with neutral atoms in the Earth's upper atmosphere. The strongest auroras are quite bright, comparable to moonlight. The aurora occur only above altitudes of 80 km and infrequently above 500 km. The average altitude for normal intensity aurora is between 110 and 200 km. This process is similar to the discharge in a neon lamp.
Altitude affects auroral color. The strong, green light originates at altitudes of 120 to 180 km. Red northern lights occur at even higher altitudes, whilst blue and violet occur mostly below 120 km. When the sun is "stormy," red colors occur at altitudes of 90 to 100 km. Entirely red northern lights sometimes may be seen, particularly at low latitudes. In earlier times, people often mistook this red light for fire on the horizon. Any particular color of the aurora depends on a specific atmospheric gas and its electrical state, and on the energy of the particle that hits the atmospheric gas. Atomic oxygen is responsible for the two main colors of green (wavelength of 557.7 nm) and red (630.0 nm). Nitrogen causes blue and deep red hues.
Auroral features come in many shapes and sizes. Tall arcs and rays start brightly 100 km above the Earth's surface and extend upward along its magnetic field for hundreds of km. These arcs or curtains can be as thin as 100 meters while extending from horizon to horizon. Auroral arcs can nearly stand still and then, as though a hand has been run along a tall curtain, the aurora will begin to dance and turn. After midnight, the aurora can take on a patchy appearance and the patches often blink on and off once every 10 seconds or so until dawn.
Most of the auroral features are greenish yellow but sometimes the tall rays will turn red at their tops and along their lower edge. On rare occasions, sunlight will hit the top part of the auroral rays creating a faint blue color. On very rare occasions (once every 10 years or so) the aurora can be a deep blood red color from top to bottom. In addition to producing light, the energetic auroral particles deposit heat. The heat is dissipated by infrared radiation or transported away by strong winds in the upper atmosphere.
What are the colors of the aurora?
The Sun radiates all visible colors, which is why sunlight appears white. The spectrum of visible light associated with the aurora is much narrower. The aurora is caused by particles of the solar wind colliding with atmospheric atoms and ions. The atmosphere consists mainly of nitrogen and oxygen, which when hit, emit characteristic colors. This is the spectrum of colors emitted by the various atoms in Earth's outer atmosphere:
