  By measuring the geometrical properties of the coronal mass ejection (CME)
flux rope and the leading shock observed on 2010 June 13 by the Solar Dynamics
Observatory (SDO) mission's Atmospheric Imaging Assembly (AIA) we determine the
Alfv\'en speed and the magnetic field strength in the inner corona at a
heliocentric distance of ~ 1.4 Rs. The basic measurements are the shock
standoff distance (deltaR) ahead of the CME flux rope, the radius of curvature
of the flux rope (Rc), and the shock speed. We first derive the Alfv\'enic Mach
number (M) using the relationship, deltaR/Rc = 0.81[(gamma-1) M^2 +
2]/[(gamma+1)(M^2-1)], where gamma is the only parameter that needed to be
assumed. For gamma =4/3, the Mach number declined from 3.7 to 1.5 indicating
shock weakening within the field of view of the imager. The shock formation
coincided with the appearance of a type II radio burst at a frequency of ~300
MHz (harmonic component), providing an independent confirmation of the shock.
The shock compression ratio derived from the radio dynamic spectrum was found
to be consistent with that derived from the theory of fast mode MHD shocks.
From the measured shock speed and the derived Mach number, we found the
Alfv\'en speed to increase from ~140 km/s to 460 km/s over the distance range
1.2 to 1.5 Rs. By deriving the upstream plasma density from the emission
frequency of the associated type II radio burst, we determined the coronal
magnetic field to be in the range 1.3 to 1.5 G. The derived magnetic field
values are consistent with other estimates in a similar distance range. This
work demonstrates that the EUV imagers, in the presence of radio dynamic
spectra, can be used as coronal magnetometers.
