Model for the mass fractionation in the January 6, 1997, coronal mass ejection

For the coronal mass ejection (CME) of January 6, 1997, strong element frac tionation of the heavy ions was observed at 1 AU with the Mass Time-of-Flig ht (MTOF) sensor of the Charge, Element, and Isotope Analysis System (CELIA S) on the Solar and Heliospheric Observatory (SOHO). 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 o f 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 4 compared to regular slow solar wind. We present a theoretical model with which we can reproduce the observed element fractionation The m odel assumes hot coronal loops with non-Maxwellian electron distributions a s the precursor structure of the CME on the solar surface. Diffusion perpen dicular to the magnetic field results in the preferential loss of lighter i ons from the loop, leading to mass fractionation. To quantitatively reprodu ce the fractionation process, the loops must have existed for similar to 28 hours before they became part of the CME plasma, a time that is commensura te with optical observations of loops in the active region from which the C ME was launched.