  In this paper, we analyzed a failed and a successful eruption that initiated
from the same polarity inversion line within NOAA AR 11387 on December 25,
2011. They both started from a reconnection between sheared arcades, having
distinct pre-eruption conditions and eruption details: before the failed one,
the magnetic fields of the core region had a weaker non-potentiality; the
external fields had a similar critical height for torus instability, a similar
local torus-stable region, but a larger magnetic flux ratio (of low corona and
near-surface region) as compared to the successful one. During the failed
eruption, a smaller Lorentz force impulse was exerted on the outward ejecta;
the ejecta had a much slower rising speed. Factors that might lead to the
initiation of the failed eruption are identified: 1) a weaker non-potentiality
of the core region, and a smaller Lorentz force impulse gave the ejecta a small
momentum; 2) the large flux ratio, and the local torus-stable region in the
corona provided strong confinements that made the erupting structure regain an
equilibrium state.
