  We present an observational study of the kinematics and fine structure of an
unwinding polar jet, with high temporal and spatial observations taken by the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO)
and the Solar Magnetic Activity Research Telescope (SMART). During the rising
period, the shape of the jet resembled a cylinder with helical structures on
the surface, while the mass of the jet was mainly distributed on the cylinder's
shell. In the radial direction, the jet expanded successively at its western
side. The radial expansion presented three distinct phases: the gradually
expanding phase, the fast expanding phase, and the steady phase. Each phase
lasted for about 12 minutes. The angular speed of the unwinding jet and the
twist transferred into the outer corona during the eruption are estimated to be
11.1 \times 10{-3} rad/s (period = 564 s) and 1.17 to 2.55 turns (or 2.34 to
5.1{\pi}) respectively. On the other hand, by calculating the azimuthal
component of the magnetic field in the jet and comparing the free energy stored
in the non-potential magnetic field with the jet's total energy, we find that
the non-potential magnetic field in the jet is enough to supply the energy for
the ejection. These new observational results strongly support the scenario
that the jets are driven by the magnetic twist, which is stored in the twisted
closed field of a bipole, and released through magnetic reconnection between
the bipole and its ambient open field.
