Solar causes of geomagnetic storms

190-196Two hundred ninety nine geomagnetic storms (GMSs) of moderate, moderately severe and severe type with Ap ≥ 20 have been investigated for the period 1978-99 and their possible interplanetary and solar causes are looked upon. Solar features like Hα, X-ray solar flares and active prominences and disappearing filaments (APDFs) have been found to occur more in lower heliographic latitudinal zones and produce larger number of GMSs at various locations on the earth. No significant correlation between magnitude of GMSs and importance of Hα and X-ray solar flares and APDFs has been observed. Maximum number of GMSs is associated with importance SF of each Hα and X-ray solar flares. On the basis of the statistical sampling of four putative solar causes of GMSs on the earth, the causes are identified leading to magnetic disturbance with Ap ≥ 20. The Hα, X-ray solar flares are more plausible causes of GMSs. However, a very few GMSs have been observed without being associated with either of these four solar features. The occurrence of disturbed days varies with seasons. Further, monthly distribution of disturbed days shows a cyclic variation having two peaks during the solar cycle