question: How does an aurora form?
comment: Charged particles that are emanated from the sun follow the earth's magnetic field lines when they impact it to the magnetic poles. Once there the charged particles energize the atmosphere they pass through on the way down to the ground creating plasma, essentially like a neon lamp.
passages: Early work on the imaging of the auroras was done in 1949 by the University of Saskatchewan using the SCR-270 radar.
  
  Section::::Occurrence.:Visual forms and colors.
  
  Auroras frequently appear either as a diffuse glow or as "curtains" that extend approximately in the east-west direction. At some times, they form "quiet arcs"; at others, they evolve and change constantly. These are called "active aurora".
   Section::::Interaction of the solar wind with Earth.
   An aurora is a natural light display in the sky, especially in the high latitude (Arctic and Antarctic) regions, in the form of that forms a large circle around the pole. They are caused by the collision of solar wind and charged magnetospheric particles with the high altitude atmosphere (thermosphere).
   Auroras are associated with the solar wind. The Earth's magnetic field traps its particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and the atmosphere release energy in the form of auroras appearing in large circles around the poles. Auroras are more frequent and brighter during the solar cycle's intense phase when CMEs increase the intensity of the solar wind.
  
  Section::::Effects.:Space weather.:Geomagnetic storm.
   Other types of auroras have been observed from space, e.g."poleward arcs" stretching sunward across the polar cap, the related "theta aurora", and "dayside arcs" near noon. These are relatively infrequent and poorly understood. Other interesting effects occur such as flickering aurora, "black aurora" and subvisual red arcs. In addition to all these, a weak glow (often deep red) observed around the two polar cusps, the field lines separating the ones that close through the Earth from those that are swept into the tail and close remotely.
  
  Section::::Occurrence.:Images.
   Most auroras occur in a band known as the "auroral zone", which is typically 3° to 6° wide in latitude and observed at 10° to 20° from the geomagnetic poles at all longitudes, but often most vividly around the spring and autumn equinoxes. The charged particles and solar wind are directed into the atmosphere by the Earth's magnetosphere. A geomagnetic storm expands the auroral zone to lower latitudes.
   A full understanding of the physical processes which lead to different types of auroras is still incomplete, but the basic cause involves the interaction of the solar wind with the Earth's magnetosphere. The varying intensity of the solar wind produces effects of different magnitudes but includes one or more of the following physical scenarios.
   Some O+ ions ("conics") also seem accelerated in different ways by plasma processes associated with the aurora. These ions are accelerated by plasma waves in directions mainly perpendicular to the field lines. They, therefore, start at their "mirror points" and can travel only upward. As they do so, the "mirror effect" transforms their directions of motion, from perpendicular to the field line to a cone around it, which gradually narrows down, becoming increasingly parallel at large distances where the field is much weaker.
  
  Section::::Auroral events of historical significance.
   Section::::Causes.:Auroral particles.
   Section::::Description and influences.
   Section::::The northern lights.
  
  Auroras are a product of the sun's activity; therefore the beautiful colorful lights seen at night are often caused by activities by the sun 150 million kilometers away. The sun sends out a constant flow of charged particles known as solar wind. When these winds or eruptions are pointed or directed at the Earth, they can spark up sun-induced auroras. The greater the activity of the sun, the brighter the auroras we see.
   Section::::Illuminations.
   Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.
  
  Section::::Interaction of the solar wind with Earth.:Magnetosphere.
   The solar wind and magnetosphere consist of plasma (ionized gas), which conducts electricity. It is well known (since Michael Faraday's work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts "across" (or is cut "by"), rather than "along", the lines of the magnetic field, an electric current is induced within the conductor. The strength of the current depends on a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the "direction" of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process ("the dynamo effect"), any and all conductors, solid or otherwise are so affected, including plasmas and other fluids.
   Section::::Overview.
   A solar wind of plasma particles constantly streams outward from the Sun until, at the outermost limit of the Solar System, it reaches the heliopause. As the solar wind passes the Earth, it interacts with the Earth's magnetic field (magnetosphere) and deflects the solar wind, but traps some creating the Van Allen radiation belts that envelop the Earth. The aurora are created when solar wind particles are guided by the magnetic flux lines into the Earth's polar regions where the lines the descend into the atmosphere.
  
  Section::::Specific subfields.:Planetary science.
   When a CME impacts the Earth's magnetosphere, it temporarily deforms the Earth's magnetic field, changing the direction of compass needles and inducing large electrical ground currents in Earth itself; this is called a geomagnetic storm and it is a global phenomenon. CME impacts can induce magnetic reconnection in Earth's magnetotail (the midnight side of the magnetosphere); this launches protons and electrons downward toward Earth's atmosphere, where they form the aurora.
  
  Section::::Types.:Diameter.
  
  Data mostly from the Michelson Doppler Imager instrument on SOHO, show changes in solar diameter to be about 0.001%, much less than the effect of magnetic activity changes.
  
  Section::::Types.:Flares.
   Aurora
  
  An aurora (plural: auroras or aurorae), sometimes referred to as polar lights, northern lights (aurora borealis), southern lights (aurora australis), is a natural light display in the Earth's sky, predominantly seen in the high-latitude regions (around the Arctic and Antarctic).
  
  Auroras are the result of disturbances in the magnetosphere caused by solar wind. These disturbances are regularly strong enough to alter the trajectories of charged particles in both solar wind and magnetospheric plasma. These particles, mainly electrons and protons, precipitate into the upper atmosphere (thermosphere/exosphere).
   BULLET::::2. Geomagnetic disturbance from an enhanced solar wind causes distortions of the magnetotail ("magnetic substorms"). These 'substorms' tend to occur after prolonged spells(hours) during which the interplanetary magnetic field has had an appreciable southward component. This leads to a higher rate of interconnection between its field lines and those of Earth. As a result, the solar wind moves magnetic flux (tubes of magnetic field lines, 'locked' together with their resident plasma) from the day side of Earth to the magnetotail, widening the obstacle it presents to the solar wind flow and constricting the tail on the night-side. Ultimately some tail plasma can separate ("magnetic reconnection"); some blobs ("plasmoids") are squeezed downstream and are carried away with the solar wind; others are squeezed toward Earth where their motion feeds strong outbursts of auroras, mainly around midnight ("unloading process"). A geomagnetic storm resulting from greater interaction adds many more particles to the plasma trapped around Earth, also producing enhancement of the "ring current". Occasionally the resulting modification of the Earth's magnetic field can be so strong that it produces auroras visible at middle latitudes, on field lines much closer to the equator than those of the auroral zone.
   Distortions to the Earth's magnetic field cause the phenomenon Aurora Borealis, commonly called the Northern Lights. The magnetic field stores energy given by cosmic particles known as solar wind, which causes the magnetic field lines to expand. When the lines contract, they release this energy, which can be seen as the Northern Lights.
  
  Section::::20th century.:Atmospheric influences.
  
  The Earth's climate changes over time due to the planet's atmospheric composition, the sun's luminosity, and the occurrence of catastrophic events.
   BULLET::::- Walter William Bryant wrote in his book (1920) that Tycho Brahe "seems to have been something of a homœopathist, for he recommends sulfur to cure infectious diseases "brought on by the sulphurous vapours of the Aurora Borealis."
  
  In the 1778, Benjamin Franklin theorized in his paper "Aurora Borealis, Suppositions and Conjectures towards forming an Hypothesis for its Explanation" that an aurora was caused by a concentration of electrical charge in the polar regions intensified by the snow and moisture in the air: 
   The high latitude magnetosphere is filled with plasma as the solar wind passes the Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density, and speed in the solar wind. This flow is favored by a southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines. The flow pattern of magnetospheric plasma is mainly from the magnetotail toward the Earth, around the Earth and back into the solar wind through the magnetopause on the day-side. In addition to moving perpendicular to the Earth's magnetic field, some magnetospheric plasma travels down along the Earth's magnetic field lines, gains additional energy and loses it to the atmosphere in the auroral zones. The cusps of the magnetosphere, separating geomagnetic field lines that close through the Earth from those that close remotely allow a small amount of solar wind to directly reach the top of the atmosphere, producing an auroral glow.
   Large solar flares occurred on 7 September 2005 (X17), 15 April 2001 (X14.4) and 29 October 2003 (X10), with auroras visible in mid-latitudes.
  
  Section::::History.:2000.
   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.
   Section::::The "Aurora" in circulation.
  