Geomagnetic Effects in the Dark Hemisphere Associated With Solar Flares

The sudden short lived perturbation in geomagnetic variation in the sunlit hemisphere concurrent with the occurrence of a solar flare referred to as 'Solar flare effect' (SFE) or 'crochet' is rather a well-studied aspect in the field of solar-terrestrial relations ever since it was discovered that chromospheric eruptions, short wave fade-outs (SWF's) and SFE's were all concomitant phenomena (DELLINGER, 1935, 1937a, b; FLEMING, 1936; RICHARDSON, 1936; TORRESON et al., 1936a, b). It is now known that the occurrence of SFE is a maximum around local noon and its amplitude has a local time variation similar to that of Sq field (MCNISH, 1937; NAGATA, 1952; SUBRAHMANYAM, 1964; PINTER, 1967). Evaluation of ionospheric current systems during flare times revealed that SFE is not due to a mere augmentation of normal Sq field as thought of by CHAPMAN (1937, 1961) and CHAPMAN and BARTELS (1940); and the current flowing layer responsible for SFE is below the normal dynamo current carrying region (VOLLAND and TAUBENHEIM, 1958; VELDKAMP and VAN SABBEN, 1960; YASHURA and MAEDA, 1961). An interpretation of SFE in terms of enhanced electrical conductivity due to solar X-ray flare flux enhancement in the band 1-20A was made by OHSHIO (1964) from a study of selected SFE events observed during I. G. Y. A recent study indicated that SFE is due to extra ionization produced by solar XUV (1-1000A) flare radiation and the SFE's often exhibit a composite structure consisting of a 'fast' component presumably due to EUV radiation (100-1000A) and a 'slow' component produced by soft X-rays (1-100A) (RICHMOND and VENKATESWARAN, 1971). A very recent study made by the authors using SFE's observed (on normal run magnetograms) at Kodaikanal (Lat: 10.20N; Long: 77.50E, Dip: 3.50N) showed that the amplitude of SFE is linearly correlated with peak solar X-ray flux level in the bands: 1-8A and 8-20A (monitored by SOLRAD-9) lending support to the interpretation that SFE is due to enhanced ionization produced by solar soft X-rays (SASTRI and MURTHY, 1975).

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