Dynamic Evolution of an X-shaped Structure above a Trans-equatorial   Quadrupole Solar Active Region Group
  In the solar corona, magnetic reconnection usually takes place at the singular configuration of magnetic field, in particular near a magnetic null owing to its high susceptibility to perturbations. In this Letter, we report a rare X-shaped structure, encompassing a magnetic null, above a trans-equatorial quadrupole active region group that is well observed by the Atmospheric Imaging Assembly (AIA). The observations show that this X-shaped structure is visible in all AIA EUV passbands and stably exists for days. However, possibly induced by flare activities at the northern part of the quadrupole active region group, the X-shaped structure starts to destabilize and meanwhile a jet erupted near its center at 15:05 UT on 2013 October 7. Through the non-linear force-free field modeling, we identify a magnetic null, which is above the quadrupole polarities and well corresponds to the X-shaped structure. After the jet eruption, the temperature and emission measure of the plasma near the X-shaped structure rise from 2.3 MK and 1.2x10^27 cm-5 at 15:01 UT to 5.4 MK and 3.7x10^27 cm-5 at 15:36 UT, respectively, revealed by the differential emission measure analysis, indicating that magnetic reconnection most likely takes place there to heat the plasma. Moreover, the height of the null has an increase of 10 Mm, which is most likely due to the partial opening of the field lines near the fan surface that makes the null underneath rise to seek for a new equilibrium. 
J. Q. Sun, X. Cheng, Y. Guo, M. D. Ding, and Y. Li
astro-ph.SR