Abstract:
The background
fi
eld is assumed to play a prime role in erupting structures like prominences. In the
fl
ux rope
models, the critical decay index
(
n
c
)
is a measure of the rate at which background
fi
eld intensity decreases with
height over the
fl
ux rope or erupting structure. In the real observations, the critical height of the background
fi
eld is
unknown, so a typical value of
n
c
=
1.5 is adopted from numerical studies. In this study, we determined the
n
c
of
10 prominence eruptions
(
PEs
)
. The prominence height in 3D is derived from two-perspective observations of the
Solar Dynamics Observatory
and the
Solar TErrestrial RElations Observatory
. Synoptic maps of photospheric
radial magnetic
fi
eld are used to construct the background
fi
eld in the corona. During the eruption, the height
–
time
curve of the sample events exhibits the slow- and fast-rise phases and is
fi
tted with the linear-cum-exponential
model. From this model, the onset height of fast-rise motion is determined and is considered as the critical height
for the onset of the torus instability because the erupting structure is allowed to expand exponentially provided
there is no strapping background
fi
eld. Corresponding to the critical height, the
n
c
values of our sample events are
varied to be in the range of 0.8
–
1.3. Additionally, the kinematic analysis suggests that the acceleration of PEs
associated with
fl
ares are signi
fi
cantly enhanced compared to
fl
areless PEs. We found that the
fl
are magnetic
reconnection is a more dominant contributor than the torus instability to the acceleration process during the fast-
rise phase of
fl
are-associated PEs in low corona
(
<
1.3
R
e
)
.
