  Halo coronal mass ejections (HCMEs) originating from regions close to the
center of the Sun are likely to be responsible for severe geomagnetic storms.
It is important to predict geo-effectiveness of HCMEs using observations when
they are still near the Sun. Unfortunately, coronagraphic observations do not
provide true speeds of CMEs due to the projection effects. In the present
paper, we present a new technique allowing estimate the space speed and
approximate source location using projected speeds measured at different
position angles for a given HCME (velocity asymmetry). We apply this technique
to HCMEs observed during 2001-2002 and find that the improved speeds are better
correlated with the travel times of HCMEs to Earth and with the magnitudes
ensuing geomagnetic storms.
