Magnetic field strength in the upper solar corona using white-light shock structures surrounding coronal mass ejections

To measure the magnetic field strength in the solar corona, we examined 12 fast (> 1000 km s-1) limb CMEs which show clear shock-like structures in SOHO/LASCO observations. By applying piston-shock relationship to the observed CME's standoff distance and electron density compression ratio, we estimated the Mach number, Alfven speed, and magnetic field strength in the height range 3 to 15 solar radii (Rs). We found: (1) the standoff distance observed in the solar corona is consistent with those from a magnetohydrodynamic (MHD) model and near-Earth observations; (2) the Mach number as a shock strength is in the range 1.49 to 3.52 from the standoff distance data, but when we use the compression ratio, the Mach number is in the range 1.47 to 1.90, implying that the measured density compression ratio is likely to be underestimated due to observational limit; (3) the Alfven speed ranges from 259 to 982 km s-1 and the magnetic field strength is in the range 6 to 120 mG when the standoff distance is used; (4) if we multiply the density compression ratio by a factor of 2, the Alfven speeds and the magnetic field strengths are consistent in both methods; (5) the magnetic field strengths derived from the shock parameters are similar to those of empirical models and previous estimates. This is a new attempt to measure magnetic field strength in the upper corona up to 15 Rs from coronagraph observations alone. Our observations support the idea that the diffuse structures surrounding the CME front can be interpreted as shock structures.