Beautiful lights sometimes seen in the night sky in northern and southern regions of the Earth are caused by the interaction of the solar wind - a stream of charged particles escaping the Sun - and our planet's magnetic field and atmosphere.
The Earth's magnetic field traps some of the particles and sends them on a collision course with molecules in the atmosphere. As a result of these repeated, tiny crashes, energy is released in the form of light.
Photo: Saturn's aurora taken by the Hubble Space Telescope (NASA, ESA, J. Clarke and G. Bacon)
The solar wind's assault on the Earth can be easily seen.
A stream of particles from the Sun stretches beyond the outer planets.
Measurements made by the 1962 Mariner 2 spacecraft confirmed the presence of solar wind, a stream of particles from the Sun that stretches far beyond the outer planets. The Earth's magnetic field fights a constant battle against the solar wind's atmosphere eroding effects.
Brian Cox travels to Norway to see the Northern Lights.
Professor Brian Cox travels to Norway in search of the Aurora Borealis, or Northern Lights. Astrophysicist Professor Mike Lockwood accompanies him as they wait for the dazzling display brought by the solar wind.
An aurora (plural: aurorae or auroras; from the Latin word aurora, "sunrise" or the Roman goddess of dawn) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone, which is typically in 3° to 6° latitudinal extent of geographical poles, or equivalently, 10° to 20° latitudinal extent of geomagnetic poles, and at all local times or longitudes. During a geomagnetic storm, the auroral zone expands to lower latitudes.
Aurorae are classified as diffuse or discrete. The diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye, even on a dark night. It defines the extent of the auroral zone. The discrete aurorae are sharply defined features within the diffuse aurora that vary in brightness from just barely visible to the naked eye, to bright enough to read a newspaper by at night. Discrete aurorae are usually seen in only the night sky, because they are not as bright as the sunlit sky. Aurorae occasionally occur poleward of the auroral zone as diffuse patches or arcs (polar cap arcs), which are generally invisible to the naked eye.
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621. Auroras seen near the magnetic pole may be high overhead, but from farther away, they illuminate the northern horizon as a greenish glow or sometimes a faint red, as if the Sun were rising from an unusual direction. Discrete aurorae often display magnetic field lines or curtain-like structures, and can change within seconds or glow unchanging for hours, most often in fluorescent green. The aurora borealis most often occurs near the equinoxes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.
Its southern counterpart, the aurora australis (or the southern lights), has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone. It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets. Similar to the Earth's aurora, they are visible close to the planet's magnetic poles. Modern style guides recommend that the names of meteorological phenomena, such as aurora borealis, be uncapitalized.