Authors: P. Wurz, P. Bochsler, and M.A. Lee
Reference: J. Geophys. Res. 105 (A12), (2000), 27239-27251
For the coronal mass ejection (CME) of January 6, 1997, strong element fractionation of the heavy ions was observed with the SOHO/CELIAS/MTOF instrument at 1~AU. During the passage of the CME plasma and the passage of the erupted filament, which followed the CME, a mass-dependent element fractionation was found with an enhancement of heavy elements, increasing monotonically with atomic mass. Si/O and Fe/O ratios around 0.5 were observed, which corresponds to an increase of about a factor of four compared to regular slow solar wind. We present a theoretical model with which we can reproduce the observed element fractionation. The model assumes hot coronal loops with non-Maxwellian electron distributions as the precursor structure of the CME on the solar surface. Diffusion perpendicular to the magnetic field results in the preferential loss of lighter ions from the loop, leading to mass fractionation. To quantitatively reproduce the fractionation process the loops must have existed for about 28 hours before they became part of the CME plasma, a time which is commensurate with optical observations of loops in the active region from which the CME was launched.