1. Kinematics and coronal field strength of an untwisting jet in a polar coronal hole observed by SDO/AIA H. Chen, J. Zhang, & S. Ma ILWS 2011-08-30, Beijing

2. Solar jets or surges have been studied since 1930s (Newton 1934). They usually appear as straight or slightly curved ejective structures. Solar Jets Jet-like events: spicules; Hα surges; macrospicules; coronal X-ray jets; white-light polar jets. Frequently-occurred jets may play an important role for heating the corona or accelerating the solar wind (Pontieu et al. 2007; Cirtain et al. 2007; Shibata et al. 2007). AIA 304

3. In some major jets, the helical or twisted structures are often reported (Dizer 1968; Jibben & Canfield 2004; Liu et al. 2009). In this talk, I will show our recent work of an untwisting jet based on the SDO/AIA observations. The AIA images the solar atmosphere in 10 wavelengths with 12s high temporal resolution and 1.5 arcsec high spatial resolution.

4. Observational results Time: from 06:00 to 07:00 UT on 2010 August 21. Location: at the northeast limb (E0N81) of the Sun, in a polar coronal hole.

5. A Small emerging flux loop at the jet base; Apparent transverse rotation motion during ejection;

6. 1. Kinematics of jet

7. The mean values of axial velocity, transverse velocity, angular speed, rotation period, and rotation radius:

8. 2. Estimation of coronal flux density Assumption: the jet plasma flows in the same flux tube during ejection, the magnetic flux across the transverse section of the jet would keep constant, i.e.

9. At the height of from 10 to 70 Mm above solar surface, B decrease from ~15±4 to 3±1 G. Dulk & McLean (1978) gave an empirical formula to describe the variation of B above active region we found the index of -0.84 fits our observations better.

10. Summary 2. By tracking six MFs in the jet, we studied the kinematics of jet plasma and derived their axial velocities, transverse velocities, angular speeds, rotation periods and rotation radiuses. 1. The jet showed a distinct transverse rotating motion during its ejection. The observational results appear to be consistent with an untwisting model of magnetic reconnection ( Shibata & Uchida 1986; Pariat et al. 2009; 2010 ). 3. On assumption of the magnetic flux conservation in the same flux tube, we estimate the coronal field strength in the polar coronal hole. Our results show that the coronal flux density at the heights of 10~70 Mm decreased from about 15 to 3 G. A formula of fits our estimated data well.