Ionospheric effects of solar flares. I. The statistical relationship between X-ray flares and SID's

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 Ha 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-2sec-1 (T = 2×106 °K) in the 0-8 Å band and a threshold ratio of 0–3 Å to 0–8 Å 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 Å, 0–8 Å, 2–12 Å 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 Å, 2–12 Å and 8–12 Å band is 2–3 min. The time curves of SID's are similar to those of X-rays in 0–3 Å, 0–8 Å and 2–12 Å bands but there are differences in details