Understanding the FIP Effect in Coronal Mass Ejections

The abundance patterns in coronal mass ejections (CMEs) are broadly similar to those in the slow speed solar wind and solar corona, but exhibit some subtle differences. The FIP effect, the enhancement in abundance of elements with first ionization potential (FIP) below about 10eV, is very similar, but in some cases can be a significantly stronger enhancement than the usual factor of about 3. The abundance ratio He/H is usually depleted in the solar wind, but less so in CMEs. We explore a model of element abundance fractionation originally proposed by Laming (2004), where chromospheric ions, but not neutrals, are subject to the ponderomotive force arising from Alfven wave propagation in the non uniform chromosphere. In solar conditions, this force is generally directed upwards, enhancing the coronal abundance of elements which are ionized in the chromosphere. By varying the model chromosphere and the spectrum of ionizing radiation, we investigate to what degree these abundance variations may be explained within the Alfven wave model. Work supported by NASA Grants NNG05HL39I (SOHO GI) and NNG08EK62I (STEREO GI), and by basic research funds of the Office of Naval Research.