  We report one of several homologous non-radial eruptions from NOAA active
region (AR) 11158 that are strongly modulated by the local magnetic field as
observed with the Solar Dynamic Observatory (SDO). A small bipole emerged in
the sunspot complex and subsequently created a quadrupolar flux system.
Non-linear force-free field (NLFFF) extrapolation from vector magnetograms
reveals its energetic nature: the fast-shearing bipole accumulated ~2e31 erg
free energy (10% of AR total) over just one day despite its relatively small
magnetic flux (5% of AR total). During the eruption, the ejected plasma
followed a highly inclined trajectory, over 60 degrees with respect to the
radial direction, forming a jet-like, inverted-Y shaped structure in its wake.
Field extrapolation suggests complicated magnetic connectivity with a coronal
null point, which is favorable of reconnection between different flux
components in the quadrupolar system. Indeed, multiple pairs of flare ribbons
brightened simultaneously, and coronal reconnection signatures appeared near
the inferred null. Part of the magnetic setting resembles that of a
blowout-type jet; the observed inverted-Y structure likely outlines the open
field lines along the separatrix surface. Owing to the asymmetrical
photospheric flux distribution, the confining magnetic pressure decreases much
faster horizontally than upward. This special field geometry likely guided the
non-radial eruption during its initial stage.
