Abstract
Abstract The Sudden Ionospheric Disturbances (SID's) produced in the Earth's ionosphere by enhanced solar radiation during solar flares are examined in the light of recent information on flare X-ray emission. The SID's cover the VLF-LF as well as HF effects. It is found that the occurrence probability of SID's with X-ray flares increases as the emission extends towards the shorter end of X-ray spectrum. An X-ray enhancement concurrent with both Hα and radio events is generally large and is capable of producing all SID effects including the sudden frequency deviation (SFD) and a crochet in 80 per cent of the cases. A threshold flux of 1 × 10−3 erg cm−2 sec−1 (T = 2 × 106 °K) in the 0–8 A band and a threshold ratio of 0–3 A to 0–8 A flux level of 1.5 × 10−2 is sufficient to produce a detectable SID effect. The size of an SID effect increases with X-ray enhancements in 0–3 A, 0–8 A, 2–12 A bands, but the relationship is not linear. The relaxation time of most SID effects (excluding SFD) with reference to X-ray flares in 0–8 A, 2–12 A and 8–12 A band is 2–3 min. The time curves of SID's are similar to those of X-rays in 0–3 A, 0–8 A and 2–12 A bands but there are differences in details.
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