Velocity-dependent fractionation in the solar chromosphere.

Details Velocity-dependent fractionation in the solar chromosphere. Velocity-dependent fractionation in the solar chromosphere.

Aug 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...312l..37p&link_type=abstract

Astronomy and Astrophysics, v.312, p.L37-L40

Astronomy and Astrophysics

Astrophysics

23

Sun: Abundances, Sun: Chromosphere, Solar Wind

Scientific paper

An explanation of the observed patterns of the first ionisation potential (FIP) fractionation of element abundances in the slow as well as in the fast speed solar wind is proposed. In the former the low-FIP (<10eV) elements are enriched in relation to the high-FIP elements by a factor of 4 compared to the photosphere. In the latter this separation is weaker, or vanishes at all. This velocity dependence can be understood by a simple analysis of atomic and plasma parameters in the solar chromosphere. For this purpose an appropriate layer of constant (hydrogen) density (10^16^m-3) and temperature (10^4^K) is assumed. It is permeated by ionising photons and embedded in a homogeneous vertical magnetic field. Only trace gases in a hydrogen background are investigated. Instead of solving the equations only the asymptotic behaviour is analysed. At the lower boundary a neutral gas mixture and a constant velocity of the neutrals is assumed. Thus there the differential equations for a trace gas are reduced to an algebraic system. This results in the constant total particle flux of the respective minor species. The obtained fractionation depends not only on atomic parameters, i.e. ionisation times and collision frequencies, but also on the plasma parameters, i.e. density, temperature and velocity. Using typical values for the chromosphere a mass flow velocity of about 200m/s leads to the observed fractionation in the slow wind, while 1000m/s correspond to the patterns in the high speed solar wind.

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