Geo-effective transients and their solar causes during solar cycle 23

Abstract

During nine-year period of the current solar cycle 23 from July 1996 to January 2005, geomagnetic storms (GMSs) of Intense if (Dst < -100nT), Major if (-50nT ≥ Dst ≥ -100nT) and Minor if (-20nT ≥ Dst ≥ -50nT) have been investigated. It is observed that maximum number of GMSs are associated with coronal mass ejections (CMEs) followed by individual H⍺ and X-ray solar flare events. When accumulated effect of Hα and X-ray solar flare events are considered, these solar flares are better associated with GMSs than CMEs. A significant decline in the number of Intense and Minor GMSs have been observed from 1998 to 1999, however, there is an increase in Major GMSs. On the contrary, during 1997-98, Intense and Minor GMSs have increased with the ascending phase of solar activity and Major GMSs have decreased. It is observed that an overall northern bias apparently prevails for solar flares and active prominences and disappearing filaments. Hα and X-ray solar flares occurring over the western limb of the solar disk cause larger disturbances in magnetosphere leading to occurrence of Intense GMSs, whereas, solar flares occurring on eastern limb of the solar disk lead to occurrence of Major and Minor GMSs. It is observed that coronal intensity (CI) is maximum for Minor GMSs followed by Major and Intense GMSs, whereas, mean CI is maximum for Intense GMSs followed by Major and Minor GMSs. The results show that the product of solar wind velocity (Vsw) with minimum Bz component (Bzmin) of interplanetary magnetic field (Vsw.Bzmin), product of linear velocity of CMEs (Vcme) with Bzmin (Vcme.Bzmin) along with minimum Dst of the sudden storm commencement day are the reliable indicators of intensity of GMSs.