CME earthward direction as an important geoeffectiveness indicator

Frontside halo coronal mass ejections (CMEs) are generally considered as potential candidates for producing geomagnetic storms, but there was no definite way to predict whether they will hit the Earth or not. Recently Moon et al. suggested that the degree of CME asymmetries, as defined by the ratio of the shortest to the longest distances of the CME front measured from the solar center, be used as a parameter for predicting their geoeffectiveness. They called this quantity a direction parameter, D, as it suggests how much CME propagation is directed to Earth, and examined its forecasting capability using 12 fast halo CMEs. In this paper, we extend this test by using a much larger database (486 frontside halo CMEs from 1997 to 2003) and more robust statistical tools (contingency table and statistical parameters). We compared the forecast capability of this direction parameter to those of other CME parameters, such as location and speed. We found the following results: (1) The CMEs with large direction parameters (D > 0.4) are highly associated with geomagnetic storms. (2) If the direction parameter increases from 0.4 to 1.0, the geoeffective probability rises from 52% to 84%. (3) All CMEs associated with strong geomagnetic storms (Dst < -200 nT) are found to have large direction parameters (D > 0.6). (4) CMEs causing strong geomagnetic storms (Dst < -100 nT), in spite of their northward magnetic field, have large direction parameters (D > 0.6). (5) Forecasting capability improves when statistical parameters (e.g., "probability of detection-yes" and "critical success index") are employed, in comparison with the forecast solely based on the location and speed of CMEs. These results indicate that the CME direction parameter can be an important indicator for forecasting CME geoeffectiveness.