Editing Solar flare (section)

== Effects ==
{{See also|Space weather#Effects}}
The electromagnetic radiation emitted during a solar flare propagates away from the Sun at the [[speed of light]] with [[Inverse-square law|intensity inversely proportional to the square of the distance from its source region]]. The excess [[ionizing radiation]], namely X-ray and extreme ultraviolet (XUV) radiation, is known to affect [[planetary atmosphere]]s and is of relevance to human space exploration and the search for extraterrestrial life.

Solar flares also affect other objects in the Solar System. Research into these effects has primarily focused on the [[atmosphere of Mars]] and, to a lesser extent, [[atmosphere of Venus|that of Venus]].<ref name="yan22">{{cite journal |last1=Yan |first1=Maodong |last2=Dang |first2=Tong |last3=Cao |first3=Yu-Tian |last4=Cui |first4=Jun |last5=Zhang |first5=Binzheng |last6=Liu |first6=Zerui |last7=Lei |first7=Jiuhou |title=A Comparative Study of Ionospheric Response to Solar Flares at Earth, Venus, and Mars |journal=The Astrophysical Journal |date=1 November 2022 |volume=939 |issue=1 |pages=23 |doi=10.3847/1538-4357/ac92ff |doi-access=free |bibcode=2022ApJ...939...23Y}}</ref> The impacts on other planets in the Solar System are little studied in comparison. As of 2024, research on their effects on [[Mercury (planet)|Mercury]] have been limited to modeling of the response of ions in [[Mercury's magnetic field|the planet's magnetosphere]],<ref>{{cite journal |last1=Werner |first1=A. L. E. |last2=Leblanc |first2=F. |last3=Chaufray |first3=J. Y. |last4=Modolo |first4=R. |last5=Aizawa |first5=S. |last6=Hadid |first6=L. Z. |last7=Baskevitch |first7=C. |title=Modeling the Impact of a Strong X-Class Solar Flare on the Planetary Ion Composition in Mercury's Magnetosphere |journal=Geophysical Research Letters |date=16 February 2022 |volume=49 |issue=3 |doi=10.1029/2021GL096614 |doi-access=free |bibcode=2022GeoRL..4996614W}}</ref> and their impact on [[Jupiter]] and [[Saturn]] have only been studied in the context of X-ray radiation [[back scatter]]ing off of the planets' upper atmospheres.<ref>{{cite journal |last1=Bhardwaj |first1=Anil |last2=Branduardi-Raymont |first2=G. |last3=Elsner |first3=R. F. |last4=Gladstone |first4=G. R. |last5=Ramsay |first5=G. |last6=Rodriguez |first6=P. |last7=Soria |first7=R. |last8=Waite |first8=J. H. |last9=Cravens |first9=T. E. |author-link1=Anil Bhardwaj |title=Solar control on Jupiter's equatorial X-ray emissions: 26–29 November 2003 XMM-Newton observation |journal=Geophysical Research Letters |date=February 2005 |volume=32 |issue=3 |doi=10.1029/2004GL021497 |doi-access=free |arxiv=astro-ph/0504670|bibcode=2005GeoRL..32.3S08B }}</ref><ref>{{cite journal |last1=Bhardwaj |first1=Anil |last2=Elsner |first2=Ronald F. |last3=Waite, Jr. |first3=J. Hunter |last4=Gladstone |first4=G. Randall |last5=Cravens |first5=Thomas E. |last6=Ford |first6=Peter G. |title=Chandra Observation of an X-Ray Flare at Saturn: Evidence of Direct Solar Control on Saturn's Disk X-Ray Emissions |journal=The Astrophysical Journal |date=10 May 2005 |volume=624 |issue=2 |pages=L121–L124 |doi=10.1086/430521 |doi-access=free |bibcode=2005ApJ...624L.121B |arxiv=astro-ph/0504110}}</ref>

=== 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>

=== Health ===
==== Low Earth orbit ====
For astronauts in [[low Earth orbit]], an expected radiation dose from the electromagnetic radiation emitted during a solar flare is about 0.05 [[Gray (unit)|gray]], which is not immediately lethal on its own. Of much more concern for astronauts is the [[particle radiation]] associated with solar particle events.<ref>{{cite journal |last1=Whittaker |first1=Ian |title=The invisible space killers – The dangers of space radiation from both inside and outside the solar system |url=https://www.physoc.org/magazine-articles/the-invisible-space-killers/ |journal=Physiology News Magazine |doi=10.36866/pn.117.36 |s2cid=214067105 |access-date=14 June 2022}}</ref>{{Better source needed|date=June 2022}}

==== Mars ====
The impacts of solar flare radiation on Mars are relevant to [[Exploration of Mars|exploration]] and the search for [[life on Mars|life on the planet]]. Models of its atmosphere indicate that the most energetic solar flares previously recorded may have provided acute doses of radiation that would have been almost harmful or lethal to mammals and other [[higher organism]]s on Mars's surface. Furthermore, flares energetic enough to provide lethal doses, while not yet observed on the Sun, are thought to occur and have been observed on other [[Sun-like star]]s.<ref>{{cite journal |last1=Smith |first1=David S. |last2=Scalo |first2=John |title=Solar X-ray flare hazards on the surface of Mars |journal=Planetary and Space Science |date=March 2007 |volume=55 |issue=4 |pages=517–527 |doi=10.1016/j.pss.2006.10.001 |bibcode=2007P&SS...55..517S |arxiv=astro-ph/0610091}}</ref><ref>{{cite journal |last1=Jain |first1=Rajmal |last2=Awasthi |first2=Arun K. |last3=Tripathi |first3=Sharad C. |last4=Bhatt |first4=Nipa J. |last5=Khan |first5=Parvaiz A. |title=Influence of solar flare X-rays on the habitability on the Mars |journal=Icarus |date=August 2012 |volume=220 |issue=2 |pages=889–895 |doi=10.1016/j.icarus.2012.06.011 |bibcode=2012Icar..220..889J}}</ref><ref>{{cite journal |last1=Thirupathaiah |first1=P. |last2=Shah |first2=Siddhi Y. |last3=Haider |first3=S.A. |title=Characteristics of solar X-ray flares and their effects on the ionosphere and human exploration to Mars: MGS radio science observations |journal=Icarus |date=September 2019 |volume=330 |pages=60–74 |doi=10.1016/j.icarus.2019.04.015 |bibcode=2018cosp...42E1350H}}</ref>