Editing Aurora (section)

=== Forms ===
According to Clark (2007), there are five main forms that can be seen from the ground, from least to most visible:<ref>{{Cite journal|doi=10.1016/j.endeavour.2007.07.004|title=Astronomical fire: Richard Carrington and the solar flare of 1859|journal= Endeavour|volume=31|issue=3|pages=104–109|year=2007|last1=Clark|first1=Stuart|pmid=17764743}}</ref>

[[File:Aurora shapes.jpg|thumb|Different forms]]
[[File:Magenta G5 aurora over Tuntorp, Lysekil Municipality 11.jpg|thumb|Divergence point of a coronal aurora]]
* A mild ''glow'', near the horizon. These can be close to the limit of visibility,<ref>{{Cite journal|doi=10.1016/S1364-6826(96)00113-7|title=Polar cap arcs: A review|journal=Journal of Atmospheric and Solar-Terrestrial Physics|volume=59|issue=10|page=1087|year=1997|last1=Zhu|first1=L.|last2=Schunk|first2=R. W.|last3=Sojka|first3=J. J.|bibcode=1997JASTP..59.1087Z }}</ref> but can be distinguished from moonlit clouds because stars can be seen undiminished through the glow.
* ''Patches'' or ''surfaces'' that look like clouds.
* ''Arcs'' curve across the sky.
* ''Rays'' are light and dark stripes across arcs, reaching upwards by various amounts.
* ''Coronas'' cover much of the sky and diverge from one point on it.

Brekke (1994) also described some auroras as "curtains".<ref name="a-1994">{{cite book|last1=A|first1=Brekke|last2=A|first2=Egeland|title=The Northern Lights|date=1994|publisher=Grøndahl and Dreyer, Oslo|isbn=978-82-504-2105-9|page=137}}</ref> The similarity to curtains is often enhanced by folds within the arcs. Arcs can fragment or break up into separate, at times rapidly changing, often rayed features that may fill the whole sky. These are also known as ''discrete auroras'', which are at times bright enough to read a newspaper at night.<ref name="yahnin-1997">{{Cite journal|doi=10.1007/s00585-997-0943-z|title=Magnetospheric source region of discrete auroras inferred from their relationship with isotropy boundaries of energetic particles|journal=Annales Geophysicae|volume=15|issue=8|page=943|year=1997|last1=Yahnin|first1=A. G.|last2=Sergeev|first2=V. A.|last3=Gvozdevsky|first3=B. B.|last4=Vennerstrøm|first4=S.|bibcode=1997AnGeo..15..943Y|doi-access=free }}</ref>

These forms are consistent with auroras being shaped by Earth's magnetic field. The appearances of arcs, rays, curtains, and coronas are determined by the [[Perspective (graphical)|shapes of the luminous parts of the atmosphere and a viewer's position]].<ref>{{Cite journal|doi=10.1073/pnas.3.1.1|pmid=16586674|pmc=1091158|title=Inferences concerning auroras|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=3|issue=1|pages=1–7|year=1917|last1= Thomson|first1=E.|bibcode=1917PNAS....3....1T|doi-access=free}}</ref>