It is evident that the FIP effect, regardless of the ionization process, cannot be achieved in a steady-state situation in which collisions play a dominant role in locking species together. In the case of the fast solar wind we have demonstrated that steady flow with photo-ionization will not produce the required separation since there is no obvious way of removing or adding particles of a given species, whether neutral or ionized. There is however a possibility of accounting for the depression of the helium flux if helium remains substantially neutral well above the ionizing layer and is able to escape from the sides as neutrals, without replacement (e.g. von Steiger & Geiss (1989)) This is perhaps sufficient to account for the abundance variations observed in the fast wind which appear to involve helium only. In the slow solar wind the situation is altogether different since it is inherently non-steady, even as far as abundance variations are concerned. The slow wind appears to have its origin in the mainly closed coronal streamer belt and is a consequence of transient opening and closing of the magnetic field lines. The plasma which escapes to form the slow wind may therefore come from almost any altitude (allowing gravitational settling to play some role in differentiating between species) and also from a regime which exhibits a strong FIP effect even if apparently in hydrostatic equilibrium. In fact it seems likely that this results primarily from magnetic separation of low FIP ions in partially ionized regions by upwards plasma drift as observed in the ionosphere of the Earth, especially in equatorial regions where the magnetic field is horizontal. Such a process preferentially injects low FIP ions but it must be inherently non-steady or spatially non-uniform since upwards motions are always associated with downwards motions if the plasma cannot escape directly into space.
© European Southern Observatory (ESO) 1998
Online publication: March 10, 1998