  Recently we established a fundamental mechanism of solar eruption initiation,
in which an eruption can be initiated from a bipolar field through magnetic
reconnection in the current sheet (CS) that is formed slowly in the core field
as driven by photospheric shearing motion. Here using a series of fully 3D MHD
simulations with a range of different photospheric magnetic flux distributions,
we extended this fundamental mechanism to the quadrupolar magnetic field
containing a null point above the core field, which is the basic configuration
of the classical breakout model. As is commonly believed, in such multipolar
configuration, the reconnection triggered in the CS originated at the null
point (namely, the breakout reconnection) plays the key role in eruption
initiation by establishing a positive feedback-loop between the breakout
reconnection and the expansion of the core field. However, our simulation
showed that the key of eruption initiation in such multipolar configuration
remains to be the slow formation of the CS in the sheared core rather than the
onset of fast breakout reconnection. The breakout reconnection only helps the
formation of the core CS by letting the core field expand faster, but the
eruption cannot occur when the bottom surface driving is stopped well before
the core CS is formed, even though the fast reconnection has already been
triggered in the breakout CS. This study clarified the role of breakout
reconnection and confirmed formation of the core CS as the key to the eruption
initiation in a multipolar magnetic field.
