Editing Aurora (section)

=== 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>