Sudden ionospheric disturbances (SID) are caused by solar flare enhanced X-rays in the 1 to 10 angstrom range. Solar flares can produce large increases of ionization in the D-region of the ionosphere over the daylit hemisphere of the Earth. SID's typically begin simultaneously or a few minutes after the start of a solar flare, and can have a time duration somewhat longer than the flare with a rise time more rapid than the decay time.Types of SIDs are SCNA--sudden cosmic noise absorption is a riometer-detected sudden daytime absorption increases; SEA--sudden enhancement of atmospherics are increases in signal strength on wideband equipment operated in the VLF, 10 to 50 KHz frequency range; SES--sudden enhancement of signal, observed on VLF frequencies in the 15 to 50 KHz range are nearly identical to SEA except that the receivers are narrow-band receivers designed to pick up man-made VLF transmission. Like SEA, signal strength increase is the SID indicator; SFD--sudden frequency deviation is the sudden increase of high frequency radiowaves which reaches a peak, and then decays back to the transmitting frequency; SPA--sudden phase anomaly is an abrupt shift in the phase of a radio signal on VLF frequencies received by ionospheric reflection; SWF--short wave fadeouts are abrupt decreases of radio signal strength observed at VLF frequencies.Tables of SIDs include the date; beginning, maximum and ending times; type of SID; importance on a scale of 1- to 3+ and the corresponding solar flare, if known.
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