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We present an observational study of the kinematics and fine structure of anunwinding polar jet, with high temporal and spatial observations taken by theAtmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO)and the Solar Magnetic Activity Research Telescope (SMART). During the risingperiod, the shape of the jet resembled a cylinder with helical structures onthe surface, while the mass of the jet was mainly distributed on the cylinder'sshell. In the radial direction, the jet expanded successively at its westernside. The radial expansion presented three distinct phases: the graduallyexpanding phase, the fast expanding phase, and the steady phase. Each phaselasted for about 12 minutes. The angular speed of the unwinding jet and thetwist transferred into the outer corona during the eruption are estimated to be11.1 \times 10{-3} rad/s (period = 564 s) and 1.17 to 2.55 turns (or 2.34 to5.1{\pi}) respectively. On the other hand, by calculating the azimuthalcomponent of the magnetic field in the jet and comparing the free energy storedin the non-potential magnetic field with the jet's total energy, we find thatthe non-potential magnetic field in the jet is enough to supply the energy forthe ejection. These new observational results strongly support the scenariothat the jets are driven by the magnetic twist, which is stored in the twistedclosed field of a bipole, and released through magnetic reconnection betweenthe bipole and its ambient open field.

Kinematics and Fine Structure of An Unwinding Polar Jet Observed by SDO/AIA