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We determine the coronal magnetic field strength in the heliocentric distancerange 6 to 23 solar radii (Rs) by measuring the shock standoff distance and theradius of curvature of the flux rope during the 2008 March 25 coronal massejection (CME) imaged by white-light coronagraphs. Assuming the adiabaticindex, we determine the Alfven Mach number, and hence the Alfven speed in theambient medium using the measured shock speed. By measuring the upstream plasmadensity using polarization brightness images, we finally get the magnetic fieldstrength upstream of the shock. The estimated magnetic field decreases from ~48mG around 6 Rs to 8 mG at 23 Rs. The radial profile of the magnetic field canbe described by a power law in agreement with other estimates at similarheliocentric distances.

The Strength and Radial Profile of Coronal Magnetic Field from the Standoff Distance of a CME-driven Shock