  Magnetic clouds (MCs) are transient magnetic structures giving the strongest
southward magnetic field (Bz south) in the solar wind. The sheath regions of
MCs may also carry southward magnetic field. Southward magnetic field is
responsible for causing space-weather disturbances. We report a comprehensive
analysis of MCs and Bz components in their sheath regions during 1995 to 2017.
Eighty-five percent of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during
the 23 years may reach as high as 40 nT. The MCs of strongest magnetic
magnitude and Bz south occur in the declining phase of the solar cycle. The
bipolar MCs have solar-cycle dependence in their polarity, but not in the
occurrence frequency. Unipolar MCs show solar-cycle dependence in their
occurrence frequency but not in their polarity. MCs with the highest speeds,
largest total B magnitudes and sheath Bz south are from source regions closer
to the solar disk center. About 80% of large Dst storms are caused by MC
events. The combinations of south Bz in the sheath and the south-first MCs in
close succession have given the largest storms. The solar-cycle dependence of
bipolar MCs is extended to 2017, spanning 42 years. We find that the bipolar MC
Bz polarity solar-cycle dependence is given by MCs originated from quiescent
filaments in decayed active regions and a group of weak MCs of unclear sources,
while the polarity of bipolar MCs with active-region flares always has mixed Bz
polarity without solar-cycle dependence and is therefore the least predictable
for Bz forecasting.
