Editing Solar flare (section)

=== Ionosphere ===
{{Further|Sudden ionospheric disturbance}}
[[File:Ionosphere Layers en.svg|thumb|Structure of Earth's nightside (left) and dayside (right) ionospheric sub-layers under normal conditions]]
Enhanced XUV irradiance during solar flares can result in increased [[ionization]], [[Dissociation (chemistry)|dissociation]], and [[heating]] in the [[ionosphere]]s of Earth and Earth-like planets. On Earth, these changes to the upper atmosphere, collectively referred to as ''sudden ionospheric disturbances'', can interfere with [[short-wave radio]] communication and [[global navigation satellite system]]s (GNSS) such as [[GPS]],<ref name="mitra74">{{cite book |last1=Mitra |first1=A. P. |title=Ionospheric Effects of Solar Flares |series=Astrophysics and Space Science Library |date=1974 |volume=46 |publisher=Springer |location=Dordrecht |url=https://archive.org/details/ionosphericeffec0046apmi |url-access=registration |doi=10.1007/978-94-010-2231-6 |isbn=978-94-010-2233-0 |language=en}}</ref> and subsequent expansion of the upper atmosphere can increase drag on satellites in [[low Earth orbit]] leading to [[orbital decay]] over time.<ref>{{cite web |title=The Impact of Flares |url=https://hesperia.gsfc.nasa.gov/rhessi3/mission/science/the-impact-of-flares/index.html |website=RHESSI Web Site |publisher=NASA |access-date=23 December 2021}}</ref><ref name="hayes21">{{cite journal |last1=Hayes |first1=Laura A. |last2=O’Hara |first2=Oscar S. D. |last3=Murray |first3=Sophie A. |last4=Gallagher |first4=Peter T. |title=Solar Flare Effects on the Earth's Lower Ionosphere |journal=Solar Physics |date=November 2021 |volume=296 |issue=11 |page=157 |doi=10.1007/s11207-021-01898-y |bibcode=2021SoPh..296..157H |arxiv=2109.06558}}</ref>{{Additional citation needed|date=July 2024|reason=Present sources have only passing mention of flare impacts on low Earth orbiting satellites.}}

Flare-associated XUV photons interact with and ionize neutral constituents of planetary atmospheres via the process of [[photoionization]]. The electrons that are freed in this process, referred to as ''photoelectrons'' to distinguish them from the ambient ionospheric electrons, are left with kinetic energies equal to the photon energy in excess of the [[Ionization energy|ionization threshold]]. In the lower ionosphere where flare impacts are greatest and [[transport phenomena]] are less important, the newly liberated photoelectrons lose energy primarily via [[thermalization]] with the ambient electrons and neutral species and via secondary ionization due to collisions with the latter, or so-called ''photoelectron [[impact ionization]]''. In the process of thermalization, photoelectrons transfer energy to neutral species, resulting in heating and expansion of the neutral atmosphere.<ref>{{cite journal |last1=Smithtro |first1=C. G. |last2=Solomon |first2=S. C. |title=An improved parameterization of thermal electron heating by photoelectrons, with application to an X17 flare |journal=Journal of Geophysical Research: Space Physics |date=August 2008 |volume=113 |issue=A8 |doi=10.1029/2008JA013077 |doi-access=free |bibcode=2008JGRA..113.8307S}}</ref> The greatest increases in ionization occur in the lower ionosphere where wavelengths with the greatest relative increase in irradiance—the highly penetrative X-ray wavelengths—are absorbed, corresponding to Earth's E and D layers and Mars's M<sub>1</sub> layer.<ref name="yan22" /><ref name="mitra74" /><ref>{{cite journal |last1=Fallows |first1=K. |last2=Withers |first2=P. |last3=Gonzalez |first3=G. |title=Response of the Mars ionosphere to solar flares: Analysis of MGS radio occultation data |journal=Journal of Geophysical Research: Space Physics |date=November 2015 |volume=120 |issue=11 |pages=9805–9825 |doi=10.1002/2015JA021108 |doi-access=free |bibcode=2015JGRA..120.9805F}}</ref><ref>{{cite journal |last1=Thiemann |first1=E. M. B. |last2=Andersson |first2=L. |last3=Lillis |first3=R. |last4=Withers |first4=P. |last5=Xu |first5=S. |last6=Elrod |first6=M. |last7=Jain |first7=S. |last8=Pilinski |first8=M. D. |last9=Pawlowski |first9=D. |last10=Chamberlin |first10=P. C. |last11=Eparvier |first11=F. G. |last12=Benna |first12=M. |last13=Fowler |first13=C. |last14=Curry |first14=S. |last15=Peterson |first15=W. K. |last16=Deighan |first16=J. |title=The Mars Topside Ionosphere Response to the X8.2 Solar Flare of 10 September 2017 |journal=Geophysical Research Letters |date=28 August 2018 |volume=45 |issue=16 |pages=8005–8013 |doi=10.1029/2018GL077730 |doi-access=free |bibcode=2018GeoRL..45.8005T}}</ref><ref name="lollo12">{{cite journal |last1=Lollo |first1=Anthony |last2=Withers |first2=Paul |last3=Fallows |first3=Kathryn |last4=Girazian |first4=Zachary |last5=Matta |first5=Majd |last6=Chamberlin |first6=P. C. |title=Numerical simulations of the ionosphere of Mars during a solar flare |journal=Journal of Geophysical Research: Space Physics |date=May 2012 |volume=117 |issue=A5 |doi=10.1029/2011JA017399 |doi-access=free |bibcode=2012JGRA..117.5314L}}</ref>

==== Radio blackouts ====
{{See also|Communications blackout#Space weather}}
The temporary increase in ionization of the daylight side of Earth's atmosphere, in particular the D layer of the [[ionosphere]], can interfere with short-wave radio communications that rely on its level of ionization for [[skywave]] propagation. Skywave, or skip, refers to the propagation of radio waves reflected or refracted off of the ionized ionosphere. When ionization is higher than normal, radio waves get degraded or completely absorbed by losing energy from the more frequent collisions with free electrons.<ref name="NOAAflare" /><ref name="mitra74" />

The level of ionization of the atmosphere correlates with the strength of the associated solar flare in soft X-ray radiation. The [[Space Weather Prediction Center]], a part of the United States [[National Oceanic and Atmospheric Administration]], classifies radio blackouts by the peak soft X-ray intensity of the associated flare.
{| class="wikitable"
|-
! Classification !! Associated<br>SXR class !! Description<ref name="SWPCscales" />
|-
| R1 || M1 || Minor radio blackout
|-
| R2 || M5 || Moderate radio blackout
|-
| R3 || X1 || Strong radio blackout
|-
| R4 || X10 || Severe radio blackout
|-
| R5 || X20 || Extreme radio blackout
|}

==== Solar flare effect<span class="anchor" id="Magnetic crochet"></span> ====
{{See also|Ionospheric dynamo region}}
[[File:Diurnal ionospheric current.jpg|thumb|Electric currents in Earth's dayside ionosphere can be strengthened during a large solar flare]]
During non-flaring or solar quiet conditions, [[electric current]]s flow through the ionosphere's dayside E layer [[Electromagnetic induction|inducing]] small-amplitude diurnal variations in the geomagnetic field. These ionospheric currents can be strengthened during large solar flares due to increases in [[electrical conductivity]] associated with enhanced ionization of the E and D layers. The subsequent increase in the induced geomagnetic field variation is referred to as a '''solar flare effect''' ('''sfe''') or historically as a '''magnetic crochet'''. The latter term derives from the French word {{wikt-lang|fr|crochet}} meaning ''hook'' reflecting the hook-like disturbances in magnetic field strength observed by ground-based [[magnetometer]]s. These disturbances are on the order of a few [[Tesla (unit)|nanoteslas]] and last for a few minutes, which is relatively minor compared to those induced during geomagnetic storms.<ref>{{cite web |last1=Thompson |first1=Richard |title=A Solar Flare Effect |url=https://www.sws.bom.gov.au/Educational/3/1/1 |publisher=[[Australian Bureau of Meteorology]] Space Weather Forecasting Centre |access-date=12 May 2022}}</ref><ref>{{cite journal |last1=Curto |first1=Juan José |title=Geomagnetic solar flare effects: a review |journal=Journal of Space Weather and Space Climate |date=2020 |volume=10 |pages=27 |doi=10.1051/swsc/2020027 |bibcode=2020JSWSC..10...27C |s2cid=226442270 |url=https://www.swsc-journal.org/articles/swsc/abs/2020/01/swsc190079/swsc190079.html|doi-access=free }}</ref>