This paper describes the space weather effects of a major CME which was accompanied by extremely violent events on the Sun. The signatures of the event in the interplanetary medium (IPM) sensed by Ooty Radio Telescope, the solar observations by LASCO coronagraph onboard SOHO, GOES X-ray measurements, satellite measurements of the interplanetary parameters, GPS based ionospheric measurements, the geomagnetic storm parameter Dst and ground based ionosonde data are used in the study to understand the space weather effects in the different regions of the solar-terrestrial environment. The effects of this event are compared and possible explanations attempted.

Examples of extreme events of solar wind and their effect on geomagnetic conditions are discussed here. It is found that there are two regimes of high speed solar wind streams with a threshold of ∼ 850 km s^-1. Geomagnetic activity enhancement rate (GAER) is defined as an average increase in Ap value per unit average increase in the peak solar wind velocity (Vp) during the stream. GAER was found to be different in the two regimes of high speed streams with +ve and-ve IMF. GAER is 0.73 and 0.53 for solar wind streams with +ve and -ve IMF respectively for the extremely high speed streams (< 850 km s^-1). This indicates that streams above the threshold speed with +ve IMF are 1.4 times more effective in enhancing geomagnetic activity than those with -ve IMF. However, the high speed streams below the threshold with -ve IMF are 1.1 times more effective in enhancing geomagnetic activity than those with +ve IMF. The violent solar activity period (October–November 2003) of cycle 23 presents a very special case during which many severe and strong effects were seen in the environment of the Earth and other planets; however, the z-component of IMF (Bz) is mostly positive during this period. The most severe geomagnetic storm of this cycle occurred when Bz was positive.

Coronal Mass Ejections (CMEs) are important phenomena in coronal dynamics causing interplanetary signatures (ICMEs). They eject large amounts of mass and magnetic fields into the heliosphere, causing major geomagnetic storms and interplanetary shocks. Geomagnetic storms are often characterized by abrupt increases in the northward component of the earth’s field, called sudden commencements (SSC) followed by large decreases of the magnetic field and slow recovery to normal values. The SSCs are well correlated with IP shocks. Here a case study of 10–15 February 2000 and also the statistical study of CME events observed by IPS array, Rajkot, during the years 2000 to 2003 and Radio Astronomy Center, Ooty are described. The geomagnetic storm index Dst, which is a measure of geo-effectiveness, is shown to be well correlated with normalized scintillation index `g', derived from Ooty Radio Telescope (ORT) observations.