Aurora is a natural light display in Earth’s sky, predominantly seen in high-latitude regions (around the Arctic and Antarctic). Auroras display dynamic patterns of brilliant lights that appear as curtains, rays, spirals or dynamic flickers covering the entire sky.
Auroras are the result of disturbances in the magnetosphere caused by solar wind. These disturbances alter the trajectories of charged particles in the magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere).
Auroras result from emissions of photons in Earth’s upper atmosphere, above 80 km (50 mi), from ionized nitrogen atoms regaining an electron, and oxygen atoms and nitrogen based molecules returning from an excited state to ground state.
They are ionized or excited by the collision of particles precipitated into the atmosphere. Both incoming electrons and protons may be involved. Excitation energy is lost within the atmosphere by the emission of a photon, or by collision with another atom or molecule:
- Oxygen emissions ar e green or orange-red, depending on the amount of energy absorbed.
- Nitrogen emissions are blue, purple or red; blue and purple if the molecule regains an electron after it has been ionized, red if returning to ground state from an excited state.
The resulting ionization and excitation of atmospheric constituents emit light of varying colour and complexity. The form of the aurora, occurring within bands around both polar regions, is also dependent on the amount of acceleration imparted to the precipitating particles.
Most auroras occur in a band known as the “auroral zone”, which is typically 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles at all local times (or longitudes), most clearly seen at night against a dark sky. A region that currently displays an aurora is called the “auroral oval”, a band displaced by the solar wind towards the night side of Earth.
There can be also auroras on other planets in the Solar System.
Both Jupiter and Saturn have magnetic fields that are stronger than Earth’s and both have extensive radiation belts. Auroras have been observed on both gas planets, most clearly using the Hubble Space Telescope, and the Cassini and Galileo spacecraft, as well as on Uranus and Neptune.
The aurorae on Saturn seem, like Earth’s, to be powered by the solar wind. However, Jupiter’s aurorae are more complex. Jupiter’s main auroral oval is associated with the plasma produced by the volcanic moon Io, and the transport of this plasma within the planet’s magnetosphere. An uncertain fraction of Jupiter’s aurorae are powered by the solar wind.
In addition, the moons, especially Io, are also powerful sources of aurora. These arise from electric currents along field lines (“field aligned currents”), generated by a dynamo mechanism due to the relative motion between the rotating planet and the moving moon.
Auroras have also been observed on Venus and Mars. Venus has no magnetic field and so Venusian auroras appear as bright and diffuse patches of varying shape and intensity, sometimes distributed over the full disc of the planet. A Venusian aurora originates when electrons from the solar wind collide with the night-side atmosphere.