Magnetic field strength measured from the standoff distance of a CME-driven shock on 2012 January 27

We report on the properties of the coronal mass ejection (CME) on 2012 January 27 and the magnetic environment into which the CME-driven shocks propagate. The CME erupted from near the northwest limb (N27W71) and was associated with the X1.7 flare at 17:37 UT. The CME-driven shock was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on Solar and Heliospheric Observatory (SOHO) and the COR1 and COR2 coronagraphs on Solar Terrestrial Relations Observatory (STEREO). Using the shock standoff distance technique (Gopalswamy and Yashiro, 2011), we obtained the Alfven Mach number at various heights from the standoff distance and the curvature of the CME flux rope. From the shock speed measured by the coronagraph observations, the local solar wind speed derived from the empirical relation (Sheeley et al 1997), and the obtained Alfven Mach number, we were able to derive the local Alfven speed (Va in km/s). The plasma density profile (n in cm^-3) was measured from the polarized brightness of the corona observed by SOHO and STEREO. Finally the magnetic field strength (B in Gauss) upstream of the shock is derived by the relation, B=4.6 x 10^-7 n^0.5 Va. By combining all observations, we obtained the magnetic field strength profile as B=0.669 r^-1.59, where r is in solar radius. The obtained profile is steeper than what was found for 2008 (r^-1.23; Gopalswamy and Yashiro 2011). The magnetic environment information is important for understanding the particle acceleration.