  Numerical simulations suggest that kink and torus instabilities are two
potential contributors to the initiation and prorogation of eruptive events. A
magnetic parameter named decay index (i.e., the coronal magnetic gradient of
the overlying fields above the eruptive flux ropes) could play an important
role in controlling kinematics of eruptions. Previous studies have identified a
threshold range of the decay index that distinguishes between eruptive and
confined configurations. Here we advance the study by investigating if there is
a clear correlation between the decay index and CME speed. 38 CMEs associated
with filament eruptions and/or two-ribbon flares are selected using the Halpha
data from the Global Halpha Network. The filaments and flare ribbons observed
in Halpha associated with the CMEs help to locate the magnetic polarity
inversion line, along which the decay index is calculated based on the
potential field extrapolation using MDI magnetograms as boundary conditions.
The speeds of CMEs are obtained from the LASCO C2 CME catalog available online.
We find that the mean decay index increases with CME speed for those CMEs with
a speed below 1000 km/s, and stays flat around 2.2 for the CMEs with higher
speeds. In addition, we present a case study of a partial filament eruption, in
which the decay indexes show different values above the erupted/non-erupted
part.
