  We present an investigation of the polar crown prominence that erupted on
2012 March 12. This prominence is observed at the southeast limb by SDO/AIA
(end-on view) and displays a quasi vertical-thread structure. Bright
U-shape/horn-like structure is observed surrounding the upper portion of the
prominence at 171 angstrom before the eruption and becomes more prominent
during the eruption. The disk view of STEREO-B shows that this long prominence
is composed of a series of vertical threads and displays a half loop-like
structure during the eruption. We focus on the magnetic support of the
prominence vertical threads by studying the structure and dynamics of the
prominence before and during the eruption using observations from SDO and
STEREO-B. We also construct a series of magnetic field models (sheared arcade
model, twisted flux rope model, and unstable model with hyperbolic flux tube
(HFT)). Various observational characteristics appear to be in favor of the
twisted flux rope model. We find that the flux rope supporting the prominence
enters the regime of torus instability at the onset of the fast rise phase, and
signatures of reconnection (post-eruption arcade, new U-shape structure, rising
blobs) appear about one hour later. During the eruption, AIA observes dark
ribbons seen in absorption at 171 angstrom corresponding to the bright ribbons
shown at 304 angstrom, which might be caused by the erupting filament material
falling back along the newly reconfigured magnetic fields. Brightenings at the
inner edge of the erupting prominence arcade are also observed in all AIA EUV
channels, which might be caused by the heating due to energy released from
reconnection below the rising prominence.
