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 10 fast (⩾1000 km s−1) limb coronal mass ejections(CMEs) that show clear shock structures in Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph images. By applying the piston–shock relationship to the observed CME's standoff distance and electron density compression ratio, we estimated the Mach number, Alfvén speed, and magnetic field strength in the height range 3–15 solar radii (Rs). The main results from this study are as follows: (1) the standoff distance observed in the solar corona is consistent with those from a magnetohydrodynamic model and near-Earth observations; (2) the Mach number as a shock strength is in the range 1.49–3.43 from the standoff distance ratio, but when we use the density compression ratio, the Mach number is in the range 1.47–1.90, implying that the measured density compression ratio is likely to be underestimated owing to observational limits; (3) the Alfvén speed ranges from 259 to 982 km s−1 and the magnetic field strength is in the range 6–105 mG when the standoff distance is used; (4) if we multiply the density compression ratio by a factor of two, the Alfvén speeds and the magnetic field strengths are consistent in both methods; and (5) the magnetic field strengths derived from the shock parameters are similar to those of empirical models and previous estimates.