A sudden ionospheric disturbance (SID) is any one of several ionospheric perturbations, resulting from abnormally high ionization/ plasma density in the D region of the ionosphere 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 (HF) ranges, and as a result often interrupts or interferes with telecommunications systems. [1]
The Dellinger effect, or sometimes Mögel–Dellinger effect, is another name for a sudden ionospheric disturbance. [2] The effect was discovered by John Howard Dellinger around 1935 and also described by the German physicist Hans Mögel (1900-1944) in 1930. [3] [4] The fadeouts are characterized by sudden onset and a recovery that takes minutes or hours.
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 is absorbed by atmospheric particles, raising them to excited states and knocking electrons free in the process of photoionization. The low altitude ionospheric layers ( 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. [5] 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". [6]
Short wave radio waves (in the HF range) are absorbed by the increased particles in the low altitude D-region of the 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 ( 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.
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