FIP fractionation: Theory

In this review, the main models of ion-neutral frationation leading to an e nhancement of the low FIP to high FIP abundance ratio in the corona or in t he solar wind, are presented. Models based on diffusion parallel to the mag netic field are discussed; they are highly dependent on the boundary condit ions. The magnetic field, that naturally separates ions from neutrals movin g perpendicular to the field lines direction, when the ion-neutral frequenc y becomes lower than the ion gyrofrequency, is expected to play an active r ole in the ion-neutral separation. It is then suggested that ion-neutral fr actionation is linked to the formation of the solar chromosphere, i.e. in m agnetic flux-tubes at a temperature between 4000 and 6000 K.