Editing Aurora (section)

=== Changes with time ===
[[File:Keogram explainer.gif|thumb|Construction of a [[keogram]] from one night's recording by an all-sky camera, 6/7 September 2021. Keograms are commonly used to visualize changes in auroras over time.]]
Auroras change with time. Over the night they begin with glows and progress toward coronas, although they may not reach them. They tend to fade in the opposite order.<ref name="a-1994" /> Until about 1963, it was thought that these changes were due to the rotation of the Earth under a pattern fixed with respect to the Sun. Later, it was found by comparing all-sky films of auroras from different places (collected during the [[International Geophysical Year]]) that they often undergo global changes in a process called [[auroral substorm]]. They change in a few minutes from quiet arcs all along the auroral oval to active displays along the dark side and after 1–3 hours they gradually change back.<ref>{{cite book|last1=T.|first1=Potemra|last2=S.-I.|first2=Akasofu|title=Magnetospheric Substorms|date=1991|publisher=American Geophysical Union |location=Washington, D.C.|isbn=0-87590-030-5|page=5}}</ref> Changes in auroras over time are commonly visualized using [[keogram]]s.<ref>{{cite web|url=http://blog.aurorasaurus.org/?p=1229|title=Eyes on the Aurora, Part 2: What is a Keogram?|website=Aurorasaurus|date=9 September 2020|accessdate=26 February 2022|archive-date=24 February 2022|archive-url=https://web.archive.org/web/20220224164140/http://blog.aurorasaurus.org/?p=1229|url-status=live}}</ref>

At shorter time scales, auroras can change their appearances and intensity, sometimes so slowly as to be difficult to notice, and at other times rapidly down to the sub-second scale.<ref name="yahnin-1997" /> The phenomenon of pulsating auroras is an example of intensity variations over short timescales, typically with periods of 2–20 seconds. This type of aurora is generally accompanied by decreasing peak emission heights of about 8&nbsp;km for blue and green emissions and above-average solar wind speeds ({{circa|500{{nbsp}}km/s}}).<ref>{{Cite journal|last1=Partamies|first1=N.|last2=Whiter|first2=D.|last3=Kadokura|first3=A.|last4=Kauristie|first4=K.|last5=Tyssøy|first5=H. Nesse|last6=Massetti|first6=S.|last7=Stauning|first7=P.|last8=Raita|first8=T.|date=2017|title=Occurrence and average behavior of pulsating aurora|journal=Journal of Geophysical Research: Space Physics|language=en|volume=122|issue=5|pages=5606–5618|doi=10.1002/2017JA024039|bibcode=2017JGRA..122.5606P|s2cid=38394431|issn=2169-9402|url=http://urn.fi/urn:nbn:fi-fe2019092429533|access-date=7 December 2019|archive-date=12 May 2024|archive-url=https://web.archive.org/web/20240512165123/https://oulurepo.oulu.fi/handle/10024/24149|url-status=live}}</ref>