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A '''field-aligned irregularity''' ('''FAI''') is an [[anisotropic]] (different values when measured in different directions) perturbation of [[Plasma (physics)|plasma]] density associated with [[magnetic field]]s. FAIs are often thought of in the context of the Earth's [[ionosphere]] where several natural processes generate FAIs in the [[E region|E-region]] and [[F region|F-region]].<ref>{{cite journal |last1=Liu |first1=Yi |last2=Zhou |first2=Chen |last3=Xu |first3=Tong |last4=Tang |first4=Qiong |last5=Deng |first5=ZhongXin |last6=Chen |first6=GuanYi |last7=Wang |first7=ZhuangKai |title=Review of ionospheric irregularities and ionospheric electrodynamic coupling in the middle latitude region |journal=Earth and Planetary Physics |date=25 September 2021 |volume=5 |issue=4 |pages=462–482 |doi=10.26464/epp2021025 |bibcode=2021E&PP....5..462L |doi-access=free }}</ref>

They occur at 50 and 140 [[MHz]], where [[electrons]] associated with the event become vertically aligned with Earth's [[magnetic field]]. FAI may occur for several hours after it starts. Optimum times for observance appear to be between 8 PM and midnight.

== References ==
{{reflist}}

[[Category:Ionosphere]]

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Field-aligned irregularity - Revision history

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