Notes on Geo-effectiveness of CMEs prepared by Xuepu Zhao As shown by the coupling function (~ VBs) between the solar wind and geomagnetosphere, the solar wind speed and the duration and intensity of Bs events are primary factors that cause intense geomagnetic storms. Since solar energetic particles are believed to be accelerated by shocks generated by CME's ejecta, the CME speed is the major factor for causing SEPs. Thus geoeffective CMEs must be 1) Earth-intercepting, 2) having high speed, 3) are able to generate long-duration, strong-intensity Bs events. 1. Present state of knowledge wrt predicting geoeffectiveness 1.1 Earth-intercepting CMEs No observations exist for definitely identifying Earth-intercepting CMEs. Some surface activities near the disk center, such as disappearing filaments, long-duration flares, SXT-arcade formation, SXT or EIT-dimming, SXR sigmoidal structures, and EIT waves have been suggested to be the candidate of Earth-intercepting CMEs. The fronside full-halo CMEs are most reliable candidate of Earth-intercepting CMEs though some Earth-intercepting CMEs may not be able to be observed being full-halo CMEs. It has been shown that frontside full-halo CMEs near solar minimum phase have one-to-one correspodence with geomagnetic storms (Webb et al., 2000), but near maximum phase only about 50% of frontside full halo CMEs are able to generate geomagnetic storms (Zhao and Webb., 2000). The halo shape change and the halo expansion as increasing time depend on the central location, the angular width and the radial speed of CMEs. By assuming constant angular width we have developed a code to match the observed halo shape and its time variation and to determine the properties of the frontside halo CMEs (Zhao et al., 2001). 1.2 The speed of Earth-intercepting CMEs No reliable way exists so far for determing the speed of Earth-intercepting CMEs. Some estimate of the radial speed is based on the estimate of the halo expasion speed for halo CMEs on the sky plane and on the assumption of the angular width of the halo CMEs. By matching the halo shape change and the halo expansion as increasing time, the halo CME model can be used to determine the accelaration and the radial speed on the basis of the LASCO observations of full-halo CMEs (Zhao et al., 2001). More samples are testing now. Observations have shown that for many, if not most of, CMEs, the angular width does maintain constant as the height increases. It is thus expected that the technique may be used to obtain the radial speed for many full-halo CMEs. The statistical connection between the CME speed near the solar surface and near the Earth has been found using the observations of the limb CME speed near the solar surface and its speed at Helio spacecraft (Lindsay et al., 1999). 1.3 The duration and intensity of Bs events Long-duration strong-intensity Bs events often consist of both the driver gas (magnetic cloud) and the shock sheath Bs events (Tsurutani and Gonzalez, 1997; Zhao et al., 1993). Nearly every long-duration large-intensity Bs event is associated with an ejecta or a CME (Zhao et al., 1993). However, the opposite association, the one that is actually useful for storm predictability, is weak; only a fraction of ejecta cause Bs events. We have studied the magnetic cloud Bs events using an expanding cylindrical flux rope model. It is found that among the eight parameters of magnetic clouds, three parameters, i.e.,the cloud bulk speed and the ecliptic latitude and the strength of the cloud's central axial field vector have significant effect on the duration and intensity of the magnetic cloud Bs event and that the other parameters have certain values that occur most frequently. The three parameters and the rotation handedness of the internal field are less difficult to infer from solar observations than other parameters. By replacing the actual values that are currently unavailable from solar observations with these most frequently occurring parameter values, the duration and intensity of the magnetic cloud Bs events could be predicted using the expanding flux rope model and the four potentially available parameter values (Zhao and Hoeksema, 1998; Zhao et al., 2000). Most, if not all, of CMEs are believed to be magnetic flux ropes rooted on the coronal base and originated in helmet streamers. The orientation of CMEs is expected to be parallel to the underlying prominence. The elevation angle of CMEs' central axial field vector could be inferred using observations of the associated filament and the polarity reversal line of the large-scale photospheric magnetic field (Zhao and Hoeksema, 1997). The orientation of magnetic clouds have been shown to be parallel to the inclination of the heliospheric current sheet (Zhao and Hoeksema, 1996; Mulligan et al., 1998). It is understandable since the the inclination of the heliospheric current sheet should be parallel to the underlying filament or cavity rope. We found recently that most, if not all, of full-halo CMEs originate in helmet streamers and the center of CMEs located near the base of the heliospheric current sheet. Thus the inclination of the heliospheric current sheet where CMEs located can be used to estimate the elevation angle of the central axial field vector of CMEs (Zhao et al., 2000). 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Hoeksema, Central axial field direction in magnetic clouds and its relation to southward interplanetary magnetic field events and dependence on disappearing solar filaments, J. Geophys. Res., 103, 2077, 1998. Zhao, X.P., J.T. Hoeksema, and K. Marubashi, Magnetic cloud Bs events and their dependence on cloud parameters, J. Geophys. Res., accepted, 2000. Zhao, X.P. and D.F. Webb, Large-scale closed field regions and halo coronal mass ejections, EOS, Vol. 81, No. 48, F975, 2000. Zhao, X.P. et al., The geometric properties and the radial speed of halo CMEs, in preparation, 2001. Zhao, X.P., J.T. Hoeksema, J.T. Gosling and J.L. Phillips, Statistics of IMF Bz events, in Solar-Terrestrial Predictions, IV, Vol. 2, edited by J. Hruska et al., p. 712, Natl. Oceanic and Atmos. Admin., Boulder, Colo., 1993.