  A new kind of magnetohydrodynamic instability and waves are analyzed for a
current sheet in the presence of a small normal magnetic field component
varying along the sheet. These waves and instability are related to existence
of two gradients of the tangential and normal magnetic field components along
the normal and tangential directions with respect to the current sheet. The
current sheet can be stable or unstable if the multiplication of two magnetic
gradients is positive or negative. In the stable region, the kink-like wave
mode is interpreted as so called flapping waves observed in the Earth's
magnetotail current sheet. The kink wave group velocity estimated for the
Earth's current sheet is of the order of a few tens kilometers per second. This
is in good agreement with the observations of the flapping motions of the
magnetotail current sheet.
