Dependence of Geomagnetic Storms on Their Associated Halo CME Parameters

We compare the geoeffective parameters of halo coronal mass ejections (CMEs). We consider 50 front-side full-halo CMEs (FFH CMEs), which are from the list of Michalek, Gopalswamy, and Yashiro (Solar Phys.246, 399, 2007), whose asymmetric-cone model parameters and earthward-direction parameter were available. For each CME we use its projected velocity [V_p], radial velocity [V_r], angle between cone axis and sky plane [γ] from the cone model, earthward-direction parameter [D], source longitude [L], and magnetic-field orientation [M] of its CME source region. We make a simple linear-regression analysis to find out the relationship between CME parameters and Dst index. The main results are as follows: i) The combined parameters [(V_rD)^1/2 and V_rγ] have higher correlation coefficients [cc] with the Dst index than the other parameters [V_p and V_r]: cc=0.76 for (V_rD)^1/2, cc=0.70 for V_rγ, cc=0.55 for V_r, and cc=0.17 for V_p. ii) Correlation coefficients between V_rγ and Dst index depend on L and M; cc=0.59 for 21 eastern events [E], cc=0.80 for 29 western events [W], cc=0.49 for 17 northward magnetic-field events [N], and cc=0.69 for 33 southward magnetic-field events [S]. iii) Super geomagnetic storms (Dst≤−200 nT) only appear in the western and southward magnetic-field events. The mean absolute Dst values of geomagnetic storms (Dst≤−50 nT) increase with an order of E+N, E+S, W+N, and W+S events; the mean absolute Dst value (169 nT) of W+S events is significantly larger than that (75 nT) of E+N events. Our results demonstrate that not only do the cone-model parameters together with the earthward-direction parameter improve the relationship between CME parameters and Dst index, but also the longitude and the magnetic-field orientation of a FFH CME source region play a significant role in predicting geomagnetic storms.