  From a magnetohydrodynamic (MHD) simulation of the eruption of a prominence
hosting coronal flux rope, we carry out forward synthesis of the circular
polarization signal (Stokes V signal) of the FeXIII emission line at 1074.7 nm
produced by the MHD model as measured by the proposed COronal Solar Magnetism
Observatory (COSMO) Large Coronagraph (LC) and infer the line-of-sight magnetic
field BLOS above the limb. With an aperture of 150 cm, integration time of 12
min, and a resolution of 12 arcsec, the LC can measure a significant BLOS with
sufficient signal to noise level, from the simulated flux rope viewed nearly
along its axis with a peak axial field strength of about 10 G. The measured
BLOS is found to relate well with the axial field strength of the flux rope for
the height range of the prominence, and can discern the increase with height of
the magnetic field strength in that height range that is a definitive signature
of the concave upturning dipped field supporting the prominence. The
measurement can also detect an outward moving BLOS due to the slow rise of the
flux rope as it develops the kink instability, during the phase when its rise
speed is still below about 41 km/s and up to a height of about 1.3 solar radii.
These results suggest that the COSMO LC has great potential in providing
quantitative information about the magnetic field structure of CME precursors
(e.g. the prominence cavities) and their early evolution for the onset of
eruption.
