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We examine geoeffective directional parameters of coronal mass ejections (CMEs). We select 30 front-side halo CMEs from SOHO LASCO CMEs whose speed is larger than 1000km/s and longitude is less than ± 30°. These are thought to be the most plausible candidate of geoeffective CMEs. We examine the relation between CMEs directional parameters (Earthward direction, eccentricity, Δ distance and central angle parameter) and the minimum value of the Dst index. We have found that the Earthward direction parameter has a good correlation with the Dst index, the eccentricity parameter has a much better correlation with the Dst index. The Δ distance and central angle parameter has a poor correlation with the Dst index. It's, however, well correlated with the Dst index in very strong geomagnetic storms. Most of CMEs causing very strong storms (Dst ≤ -200nT) are found to have large Earthward direction parameter (≥ 0.6), small eccentricity, Δ distance and central angle parameters (E ≤ 0.4, ΔX and sin θ ≤ 0.2). These directional parameters are very important parameters that control the geoeffectiveness of very fast front-side halo CMEs

Comparing Directional Parameters of Very Fast Halo CMEs