A sudden ionospheric disturbance (SID) is any one of several ionospheric perturbations, resulting from abnormally high ionization/

plasma Plasma or plasm may refer to: Science * Plasma (physics), one of the four fundamental states of matter * Plasma (mineral), a green translucent silica mineral * Quark–gluon plasma, a state of matter in quantum chromodynamics Biology * Blood pla ...

density in the

D region D, or d, is the fourth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''dee'' (pronounced ), plural ''dees''. History The ...

of the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...

and caused by a solar flare and/or solar particle event (SPE). The SID results in a sudden increase in radio-wave absorption that is most severe in the upper medium frequency (MF) and lower

high frequency High frequency (HF) is the ITU designation for the range of radio frequency electromagnetic waves (radio waves) between 3 and 30 megahertz (MHz). It is also known as the decameter band or decameter wave as its wavelengths range from one to ten ...

(HF) ranges, and as a result often interrupts or interferes with telecommunications systems.

Discovery

The ''Dellinger effect'', or sometimes ''Mögel–Dellinger effect'', is another name for a sudden ionospheric disturbance. The effect was discovered by

John Howard Dellinger John Howard Dellinger (July 3, 1886 – December 28, 1962) was a noted American telecommunication engineer who discovered how solar flares caused fadeouts of short-wave radios (the Dellinger effect). Dellinger was born in Cleveland, Ohio, f ...

around 1935 and also described by the German physicist (1900-1944) in 1930. The fadeouts are characterized by sudden onset and a recovery that takes minutes or hours.

Cause

When a solar flare occurs on the Sun a blast of intense ultraviolet (UV) and x-ray (sometimes even gamma ray) radiation hits the dayside of the Earth after a propagation time of about 8 minutes. This high energy

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...

is absorbed by atmospheric particles, raising them to excited states and knocking electrons free in the process of photoionization. The low altitude ionospheric layers (

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. 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".

Effects on radio waves

Short wave radio waves (in the HF range) are absorbed by the increased particles in the low altitude D-region of the

, causing a complete blackout of radio communications. This is called a

short wave Shortwave radio is radio transmission using shortwave (SW) radio frequencies. There is no official definition of the band, but the range always includes all of the High frequency, high frequency band (HF), which extends from 3 to 30 MHz (10 ...

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 ( VLF) radio propagation. SIDs are observed and recorded by monitoring the signal strength of a distant VLF 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.

References

{{Reflist, 2

External links

AAVSO SID Monitoring Program

Further information on SID monitoring

Space Weather Monitors- Stanford SOLAR Center

Amateur SID monitoring station

NASA May 6, 2008 Ionosphere Planetary science Radio frequency propagation Space science Space weather

Discovery

Cause

Effects on radio waves

See also

References

External links