  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.
