Abstract :
Interplanetary magnetic clouds (a) preceded by a shock (b) followed by interaction region and (c) clouds without any such association, have been considered. Corresponding interplanetary plasma\field data were divided into three groups. Superposed analysis of these plasma\field data along with geomagnetic index ap have been performed. Differences in enhancement in the level of geomagnetic disturbance and its time profile in association with these three groups of clouds have been noticed. Each class has also been divided into two groups on the basis, whether the magnetic field in them turns from southward to northward or otherwise (i.e. northward to southward). Correlation analysis of ap index with various solar wind parameters (V, n, B, Bz) has also been done in the present work. The results support the hypothesis that the field magnitude may not be of basic importance but its southward component is crucial in deciding the level of geomagnetic activity. Out of the two plasma parameters of magnetic clouds (V and n), the plasma density itself does not appear to have any appreciable effect ; however the level of geomagnetic disturbance changes with solar plasma velocity. The level of geomagnetic disturbance, as measured by ap, increases as velocity increases, both during the passage of southward as well as northward field region ; however, for the linear curve obtained between ap and V, the slope of the curve is much larger during southward field condition in comparison to northward field condition. It shows that the solar wind velocity is usually a minor factor for the creation of geomagnetic storms. A moderate geomagnetic activity, apparently observed during northward field conditions and high solar wind velocity, in fact may be due to the presence of nonlinear Alfvén waves ; these waves have significant north-south fluctuations during high speed streams. Another conclusion drawn from the obtained relationship between solar wind velocity and ap index during northward and southward field conditions is that the result, although emphasizing the importance of southward field condition, puts some constraint on the half-wave rectifier model for magnetospheric disturbance.
