View source for Astronomy:Sudden ionospheric disturbance

A '''sudden ionospheric disturbance''' ('''SID''') is any one of several ionospheric perturbations, resulting from abnormally high ionization/[[Physics:Plasma|plasma]] density in the [[Astronomy:D region|D region]] of the [[Astronomy:Ionosphere|ionosphere]] and caused by a [[Astronomy:Solar flare|solar flare]] and/or [[Astronomy:Solar particle event|solar particle event]] (SPE). The SID results in a sudden increase in radio-wave absorption that is most severe in the upper [[Physics:Medium frequency|medium frequency]] (MF) and lower [[Physics:High frequency|high frequency]] (HF) ranges, and as a result often interrupts or interferes with [[Engineering:Telecommunication|telecommunication]]s systems.<ref>Federal Standard 1037C ''Glossary of Telecommunications Terms'', http://www.its.bldrdoc.gov/fs-1037/fs-1037c.htm, retrieved 2011 Dec 15</ref>
==Discovery==
The ''Dellinger effect'', or sometimes ''Mögel–Dellinger effect'', is another name for a sudden ionospheric disturbance.<ref>{{cite book
|author=Davies, Kenneth
|title=Ionospheric Radio
|isbn= 978-0-86341-186-1
|series=IEE Electromagnetic Waves Series #31
|date=1990
|publisher=Peter Peregrinus Ltd/The Institution of Electrical Engineers
|location=London, UK
|pages = 316–317
}}</ref> The effect was discovered by [[Biography:John Howard Dellinger|John Howard Dellinger]] around 1935 and also described by the German physicist {{interlanguage link|Hans Mögel|de}} (1900-1944) in 1930.<ref>{{cite journal |last1=Mögel |first1=H. |title=Über die Beziehungen zwischen Empfangsstörungen bei Kurzwellen und den Störungen des magnetischen Feldes der Erde |journal=Telefunken Zeitung |date=1930 |volume=11 |pages=14–31 |trans-title=On the relations between disturbances of shortwave reception and disturbances of the Earth's magnetic field}}</ref><ref>See:
*  {{cite journal |last1=Dellinger |first1=J. H. |title=A new cosmic phenomenon |journal=Science |date=11 October 1935 |volume=82 |issue=2128 |page=351 |doi=10.1126/science.82.2128.351.a |pmid=17733214 |s2cid=239877535 |url=https://www.science.org/doi/10.1126/science.82.2128.351.a}}
*  {{cite journal |last1=Dellinger |first1=J. H. |title=Sudden ionospheric disturbances |journal=Terrestrial Magnetism and Atmospheric Electricity |date=March 1937 |volume=42 |issue=1 |pages=49–53|doi=10.1029/TE042i001p00049 |bibcode=1937TeMAE..42...49D }}
*  {{cite journal |last1=Dellinger |first1=John Howard |title=Sudden disturbances of the ionosphere |journal=Journal of Research of the National Bureau of Standards |date=August 1937 |volume=19 |issue=2 |pages=111–141|doi=10.6028/jres.019.003 |doi-access=free }}</ref>  The fadeouts are characterized by sudden onset and a recovery that takes minutes or hours.

==Cause==
When a solar flare occurs on the [[Astronomy:Sun|Sun]] a blast of intense [[Physics:Ultraviolet|ultraviolet]] (UV) and [[Physics:X-ray|x-ray]] (sometimes even [[Physics:Gamma ray|gamma ray]]) radiation hits the dayside of the Earth after a propagation time of about 8 minutes. This high energy [[Physics:Radiation|radiation]] is absorbed by atmospheric particles, raising them to excited states and knocking [[Physics:Electron|electron]]s free in the process of [[Physics:Photoionization|photoionization]]. The low altitude ionospheric layers ([[Astronomy:D region|D region]] and E region) immediately increase in density over the entire dayside. The ionospheric disturbance enhances VLF radio propagation. Scientists on the ground can use this enhancement to detect solar flares; by monitoring the signal strength of a distant VLF transmitter, sudden ionospheric disturbances (SIDs) are recorded and indicate when solar flares have taken place.<ref>[https://web.archive.org/web/20030705045324/http://www.aavso.org/observing/programs/solar/sid.shtml AAVSO: SIDs – Sudden Ionospheric Disturbances<!-- Bot generated title -->]</ref> The small geomagnetic effect in the lower ionosphere appears as a small hook on magnetic records and is therefore called "geomagnetic crochet effect" or "sudden field effect".<ref>{{Citation | author1=Bartels, J | author2=Bartels, J., (editor.) | author3=SpringerLink (Online service) | title=Geophysik III | date=1967 | publisher=Springer Berlin Heidelberg | page=362 | isbn=978-3-642-46082-1 }}</ref>

==Effects on radio waves==
Short wave radio waves (in the [[Physics:High frequency|HF]] range) are absorbed by the increased particles in the low altitude D-region of the [[Astronomy:Ionosphere|ionosphere]], causing a complete blackout of radio communications. This is called a short wave fadeout (SWF). These fadeouts last for a few minutes to a few hours and are most severe in the equatorial regions where the Sun is most directly overhead. Although High Frequency signals suffer a fadeout because of the enhanced D-layer, the Sudden Ionospheric Disturbance enhances long wave ([[Physics:Very low frequency|VLF]]) radio propagation. SIDs are observed and recorded by monitoring the signal strength of a distant VLF [[Engineering:Transmitter|transmitter]].
A whole array of sub-classes of SIDs exist, detectable by different techniques at various wavelengths: the short-wave fadeout (SWF), the SPA (Sudden Phase Anomaly), SFD (Sudden Frequency Deviation), SCNA (Sudden Cosmic Noise Absorption), SEA (Sudden Enhancement of Atmospherics), etc.

==See also==
* [[Astronomy:Ionospheric storm|Ionospheric storm]]
* [[Astronomy:Space weather|Space weather]]

==References==
{{Reflist|2}}

==External links==
* [http://www.aavso.org/solar-sids AAVSO SID Monitoring Program]
* [http://www.aavso.org/sid-monitoring Further information on SID monitoring]
* [http://solar-center.stanford.edu/SID/ Space Weather Monitors- Stanford SOLAR Center]
* [http://sidstation.loudet.org/ Amateur SID monitoring station]
* [https://web.archive.org/web/20130207021514/http://activefrance.com/TechShed/page5.html SID monitoring using Spectrum Lab]
* [https://web.archive.org/web/20080509160911/http://science.nasa.gov/headlines/y2008/06may_carringtonflare.htm NASA - Carrington Super Flare] [[Organization:NASA|NASA]] May 6, 2008

[[Category:Ionosphere]]
[[Category:Planetary science]]
[[Category:Radio frequency propagation]]
[[Category:Space science]]
[[Category:Space weather]]

{{Sourceattribution|Sudden ionospheric disturbance}}