  It is well accepted that a magnetic flux rope (MFR) is a critical component
of many coronal mass ejections (CMEs), yet how it evolves toward eruption
remains unclear. Here we investigate the continuous evolution of a pre-existing
MFR, which is rooted in strong photospheric magnetic fields and electric
currents. The evolution of the MFR is observed by the Solar Terrestrial
Relations Observatory (STEREO) and the Solar Dynamics Observatory (SDO) from
multiple viewpoints. From STEREO's perspective, the MFR starts to rise slowly
above the limb five hours before it erupts as a halo CME on 2012 June 14. In
SDO observations, conjugate dimmings develop on the disk, simultaneously with
the gradual expansion of the MFR, suggesting that the dimmings map the MFR's
feet. The evolution comprises a two-stage gradual expansion followed by another
stage of rapid acceleration/eruption. Quantitative measurements indicate that
magnetic twist of the MFR increases from 1.0 +/- 0.5 to 2.0 +/- 0.5 turns
during the five-hour expansion, and further increases to about 4.0 turns per AU
when detected as a magnetic cloud at 1 AU two day later. In addition, each
stage is preceded by flare(s), implying reconnection is actively involved in
the evolution and eruption of the MFR. The implications of these measurements
on the CME initiation mechanisms are discussed.
