  Coronal mass ejections (CMEs) are large-scale eruptions of plasma from the
coronae of stars. Understanding the plasma processes involved in CME initiation
has applications to space weather forecasting and laboratory plasma
experiments. James et al. (Sol. Phys. 292, 71, 2017) used EUV observations to
conclude that a magnetic flux rope formed in the solar corona above NOAA Active
Region 11504 before it erupted on 14 June 2012 (SOL2012-06-14). In this work,
we use data from the Solar Dynamics Observatory to model the coronal magnetic
field of the active region one hour prior to eruption using a nonlinear
force-free field extrapolation, and find a flux rope reaching a maximum height
of 150 Mm above the photosphere. Estimations of the average twist of the
strongly asymmetric extrapolated flux rope are between 1.35 and 1.88 turns,
depending on the choice of axis, although the erupting structure was not
observed to kink. The decay index near the apex of the axis of the extrapolated
flux rope is comparable to typical critical values required for the onset of
the torus instability, so we suggest that the torus instability drove the
eruption.
