Editing Aurora (section)

== Causes ==
A full understanding of the physical processes that lead to different types of auroras is still incomplete, but the basic cause involves the interaction of the [[solar wind]] with [[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.
# A quiescent solar wind flowing past Earth's magnetosphere steadily interacts with it and can inject solar wind particles directly onto the geomagnetic field lines that are 'open', as opposed to being 'closed' in the opposite hemisphere and provide diffusion through the [[bow shock]]. It can also cause particles already trapped in the [[Van Allen radiation belt|radiation belts]] to precipitate into the atmosphere. Once particles are lost to the atmosphere from the radiation belts, under quiet conditions, new ones replace them only slowly, and the loss cone becomes depleted. In the magnetotail, however, particle trajectories seem constantly to reshuffle, probably when the particles cross the very weak magnetic field near the equator. As a result, the flow of electrons in that region is nearly the same in all directions ("isotropic") and assures a steady supply of leaking electrons. The leakage of electrons does not leave the tail positively charged, because each leaked electron lost to the atmosphere is replaced by a low-energy electron drawn upward from the [[ionosphere]]. Such replacement of "hot" electrons by "cold" ones is in complete accord with the [[second law of thermodynamics]]. The complete process, which also generates an electric ring current around Earth, is uncertain.
# Geomagnetic disturbance from an enhanced [[solar wind]] causes distortions of the [[magnetosphere|magnetotail]] ("magnetic substorms"). These 'substorms' tend to occur after prolonged spells (on the order of 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 ("[[plasmoid]]s") 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 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.
#: [[File:Moon and Aurora.jpg|thumb|[[Moon]] and aurora]]
# Acceleration of auroral charged particles invariably accompanies a magnetospheric disturbance that causes an aurora. This mechanism, which is believed to predominantly arise from strong electric fields along the magnetic field or wave-particle interactions, raises the velocity of a particle in the direction of the guiding magnetic field. The pitch angle is thereby decreased and increases the chance of it being precipitated into the atmosphere. Both electromagnetic and electrostatic waves, produced at the time of greater geomagnetic disturbances, make a significant contribution to the energizing processes that sustain an aurora. Particle acceleration provides a complex intermediate process for transferring energy from the solar wind indirectly into the atmosphere.
[[File:Aurora australis 20050911.jpg|right|thumb|Aurora australis (11 September 2005) as captured by NASA's [[IMAGE (spacecraft)|IMAGE]] satellite, digitally overlaid onto ''[[The Blue Marble]]'' composite image.

[[:File:Aurora Australis.gif|An animation]] created using the same satellite data is also available.]]
The details of these phenomena are not fully understood. However, it is clear that the prime source of auroral particles is the solar wind feeding the magnetosphere, the reservoir containing the radiation zones and temporarily magnetically trapped particles confined by the geomagnetic field, coupled with particle acceleration processes.<ref>{{cite book|last1=Burch|first1=J L|editor1-last=Akasofu S–I and Y Kamide|title=The solar wind and the Earth|date=1987|publisher=D. Reidel|isbn=978-90-277-2471-7|page=103}}</ref>

=== Auroral particles ===
The immediate cause of the ionization and excitation of atmospheric constituents leading to auroral emissions was discovered in 1960, when a pioneering rocket flight from Fort Churchill in Canada revealed a flux of electrons entering the atmosphere from above.<ref>{{cite journal|last1=McIlwain|first1=C E|title=Direct Measurement of Particles Producing Visible Auroras|journal=Journal of Geophysical Research|year=1960|volume=65|issue=9|page=2727|doi=10.1029/JZ065i009p02727|bibcode=1960JGR....65.2727M}}</ref> Since then an extensive collection of measurements has been acquired painstakingly and with steadily improving resolution since the 1960s by many research teams using rockets and satellites to traverse the auroral zone. The main findings have been that auroral arcs and other bright forms are due to electrons that have been accelerated during the final few 10,000&nbsp;km or so of their plunge into the atmosphere.<ref>{{Cite journal|doi=10.1029/JA093iA07p07441|title=Determination of auroral electrostatic potentials using high- and low-altitude particle distributions|journal=Journal of Geophysical Research|volume=93|issue=A7|page=7441|year=1988|last1=Reiff|first1=P. H.|last2=Collin|first2=H. L.|last3=Craven|first3=J. D.|last4=Burch|first4=J. L.|last5=Winningham|first5=J. D.|last6=Shelley|first6=E. G.|last7=Frank|first7=L. A.|last8=Friedman|first8=M. A.|bibcode=1988JGR....93.7441R }}</ref> These electrons often, but not always, exhibit a peak in their energy distribution, and are preferentially aligned along the local direction of the magnetic field.

Electrons are mainly responsible for diffuse and pulsating auroras have, in contrast, a smoothly falling energy distribution, and an angular (pitch-angle) distribution favouring directions perpendicular to the local magnetic field. Pulsations were discovered to originate at or close to the equatorial crossing point of auroral zone magnetic field lines.<ref>{{Cite journal|doi=10.1038/215045a0|title=Evidence for Velocity Dispersion in Auroral Electrons|journal=Nature|volume=215|issue=5096|page=45|year=1967|last1=Bryant|first1=D. A.|last2=Collin|first2=H. L.|last3=Courtier|first3=G. M.|last4=Johnstone|first4=A. D.|bibcode=1967Natur.215...45B|s2cid=4173665 }}</ref> Protons are also associated with auroras, both discrete and diffuse.

=== Atmosphere ===
Auroras result from emissions of [[photon]]s in Earth's upper [[Earth's atmosphere|atmosphere]], above {{convert|80|km|mi|abbr=on}}, from [[ionized]] [[nitrogen]] atoms regaining an electron, and [[oxygen]] atoms and [[nitrogen]]-based molecules returning from an [[excited state]] to the [[ground state]].<ref>{{cite web|title=Ultraviolet Waves|url=http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|url-status=dead|archive-url=https://web.archive.org/web/20110127004149/http://missionscience.nasa.gov/ems/10_ultravioletwaves.html|archive-date=27 January 2011}}</ref> 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: green or orange-red, depending on the amount of energy absorbed.
;[[Nitrogen]] emissions: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.

Oxygen is unusual in terms of its return to ground state: it can take 0.7 seconds to emit the 557.7&nbsp;nm green light and up to two minutes for the red 630.0&nbsp;nm emission. Collisions with other atoms or molecules absorb the excitation energy and prevent emission; this process is called [[Quenching (fluorescence)|collisional quenching]]. Because the highest parts of the atmosphere contain a higher percentage of oxygen and lower particle densities, such collisions are rare enough to allow time for oxygen to emit red light. Collisions become more frequent progressing down into the atmosphere due to increasing density, so red emissions do not have time to happen, and eventually, even green light emissions are prevented.

The change in auroral colour with altitude is therefore explained—oxygen red is predominant at high altitudes, followed by oxygen green and nitrogen blue/purple/red, then finally other hues of nitrogen blue/purple/red where particle collisions prevent oxygen from emissions. Green is the most common colour. Then comes pink, a mixture of light green and red, followed by pure red, then yellow (a mixture of red and green), and finally, pure blue.

Precipitating protons generally produce optical emissions as incident [[hydrogen]] atoms after gaining electrons from the atmosphere. Proton auroras are usually observed at lower latitudes.<ref>{{cite web|url=http://auspace.athabascau.ca/handle/2149/518|title=Simultaneous ground and satellite observations of an isolated proton arc at sub-auroral latitudes|publisher=Journal of Geophysical Research|date=2007|access-date=5 August 2015|archive-date=5 August 2015|archive-url=https://web.archive.org/web/20150805154623/http://auspace.athabascau.ca/handle/2149/518|url-status=live}}</ref>

=== Ionosphere ===
Bright auroras are generally associated with [[Birkeland current]]s (Schield et al., 1969;<ref>{{cite journal|doi=10.1029/JA074i001p00247|last1=Schield|first1=M. A.|last2=Freeman|first2=J. W.|last3=Dessler|first3=A. J.|year=1969|title=A Source for Field-Aligned Currents at Auroral Latitudes|journal=Journal of Geophysical Research|volume=74|issue=1|pages=247–256|bibcode=1969JGR....74..247S}}</ref> Zmuda and Armstrong, 1973<ref>{{cite journal|doi=10.1029/JA078i028p06802|last1=Armstrong|first1=J. C.|last2=Zmuda|first2=A. J.|year=1973|title=Triaxial magnetic measurements of field-aligned currents at 800 kilometers in the auroral region: Initial results|journal=Journal of Geophysical Research|volume=78|issue=28|pages=6802–6807|bibcode=1973JGR....78.6802A}}</ref>), which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125&nbsp;km); the rest ("region 2") detours, leaving again through field lines closer to the equator and closing through the "partial ring current" carried by magnetically trapped plasma. The ionosphere is an [[Ohm's law|ohmic conductor]], so some consider that such currents require a driving voltage, which an, as yet unspecified, dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms. In another interpretation, the currents are the direct result of electron acceleration into the atmosphere by wave/particle interactions.

Ionospheric resistance has a complex nature and leads to a secondary [[Hall current]] flow. Due to physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral [[electrojet]]. An auroral electrojet index (measured in nanotesla) is regularly derived from ground data and serves as a general measure of auroral activity. [[Kristian Birkeland]]<ref>{{cite book|last=Birkeland|first=Kristian|title=The Norwegian Aurora Polaris Expedition 1902–1903|date=1908|publisher=H. Aschehoug & Co.|location=New York: Christiania (Oslo)|page=720|url=https://archive.org/details/norwegianaurorap01chririch}} out-of-print, full text online</ref> deduced that the currents flowed in the east–west directions along the auroral arc, and such currents, flowing from the dayside toward (approximately) midnight were later named "auroral electrojets" (see also [[Birkeland current]]s). The ionosphere can contribute to the formation of auroral arcs via the [[feedback]] instability under high ionospheric resistance conditions, observed at night time and in the dark Winter hemisphere.<ref>{{cite journal|last1=Pokhotelov|first1=D.|last2=Lotko|first2=W. |last3=Streltsov|first3=A.V.|title= Effects of the seasonal asymmetry in ionospheric Pedersen conductance on the appearance of discrete aurora | journal=Geophys. Res. Lett. |date=2002|volume=29|issue=10|pages=79-1-79-4|doi=10.1029/2001GL014010|bibcode=2002GeoRL..29.1437P |s2cid=123637108 |doi-access=free}}</ref>