Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...301..261m&link_type=abstract
Astronomy and Astrophysics, v.301, p.261
Astronomy and Astrophysics
Astrophysics
59
Sun: Abundances, Sun: Chromosphere, (Sun:) Solar Wind
Scientific paper
A new mechanism to explain the observed first ionization potential (FIP) fractionation of coronal and solar wind element abundances is proposed. By the FIP fractionation, low-FIP (<10eV) elements are enriched in the solar corona and solar wind relative to the photosphere. This effect has been located earlier to take place in the chromosphere, at densities of N =~10^16^-10^18^m^-3^ and a temperature of T=~10^4^K, where a large fraction of the gas is still neutral. We discuss a new mechanism for the FIP fractionation in the form of a stationary diffusion model. It is based on a weakly stratified chromospheric layer of constant density of the element hydrogen and constant temperature. This layer is permeated everywhere by ionizing photons and contains a homogeneous vertical magnetic field. Otherwise, our model does not invoke any particular geometry or special set up of the system. It is thus founded solely on robust and well understood atomic collisional physics. Technically, a boundary value problem of four 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. By splitting the system into a main gas (hydrogen) and trace gases (16 elements from He to Xe), an analytical solution for the former can be found. This then serves as a background for the numerical integration of each trace gas system, for which we consider collisions between its atoms and ions with the main gas, i.e. protons and hydrogen. Boundary conditions are such that the gas is neutral at the bottom of the slab and fully ionized at its top, as a result of irradiation by the solar coronal EUV. Starting with a uniform density at the bottom of the layer, we find that, after a few hydrogen diffusion lengths, each minor species asymptotically approaches a constant density. The ratios of these density values to some reference trace element reproduce the observed FIP fractionation pattern of heavy elements remarkably well. The step between low-FIP and high-FIP element abundances is about a factor of 5, and He is somewhat depleted relative to the high-FIP elements, in agreement with the observations. The model fractionation pattern proves to be remarkably stable against changes in the external parameters (within reasonable chromospheric values), particularly N and T.
Bochsler Peter
Marsch Eckart
von Steiger Rudolf
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