  Solar cycle 23 witnessed the observation of hundreds of halo coronal mass
ejections (CMEs), thanks to the high dynamic range and extended field of view
of the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and
Heliospheric Observatory (SOHO) mission. More than two thirds of halo CMEs
originating on the front side of the Sun have been found to be geoeffective
(Dst =< -50 nT). The delay time between the onset of halo CMEs and the peak of
ensuing geomagnetic storms has been found to depend on the solar source
location (Gopalswamy et al., 2007). In particular, limb halo CMEs (source
longitude > 45deg) have a 20% shorter delay time on the average. It was
suggested that the geomagnetic storms due to limb halos must be due to the
sheath portion of the interplanetary CMEs (ICMEs) so that the shorter delay
time can be accounted for. We confirm this suggestion by examining the sheath
and ejecta portions of ICMEs from Wind and ACE data that correspond to the limb
halos. Detailed examination showed that three pairs of limb halos were
interacting events. Geomagnetic storms following five limb halos were actually
produced by other disk halos. The storms followed by four isolated limb halos
and the ones associated with interacting limb halos, were all due to the sheath
portions of ICMEs.
