Magnetic Field Strength in the Upper Solar Corona Using White-light Shock Structures Surrounding Coronal Mass Ejections
Abstract
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 15Rs. Main results from this study are: (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 projection effects; (3) the Alfven speeds range from 259 to 982 km s-1 and the magnetic field strength is in the range 0.04 to 0.35 G when the standoff distance is used; (4) if we multiply the compression ratio by a factor of 2, the Alfven speeds and the magnetic field strengths are consistent in both methods; (5) the derived magnetic field strengths in the inner corona are similar to those of empirical models but noticeably higher in the upper corona. This is a new attempt to measure magnetic field strength from coronagraph observation alone. These observations are consistent with the idea that the diffuse structures surrounding the CME front can be interpreted as shock structures.
- Publication:
-
AAS/Solar Physics Division Abstracts #42
- Pub Date:
- May 2011
- Bibcode:
- 2011SPD....42.2306K