DIFFUSIVE FRACTIONATION IN THE CHROMOSPHERE

A new mechanism for the FIP fractionation in the solar wind in the for m of a stationary diffusion model is proposed. It is based on a weakly stratified chromospheric layer of constant density and temperature, p ermeated everywhere by ionizing photons and a homogeneous magnetic fie ld. Our model does not invoke any particular geometry or special set u p of the system and is founded solely on robust and well understood at omic collisional physics. Technically, a boundary value problem of fou r coupled differential equations is solved for each chemical element, i.e. a continuity equation and a momentum equation for both atoms and singly ionized particles. For the main gas (hydrogen), an analytical s olution can be found. This then serves as a background for the numeric al integration of each trace gas system (several elements from He to F e). We find that, after a few hydrogen diffusion lengths, each minor s pecies asymptotically approaches a constant density. The ratios of the se density values to some reference element reproduce the observed FIP fractionation pattern remarkably well.