Astron. Astrophys. 333, 1069-1081 (1998)

Hydrogen and helium in the solar chromosphere:
a background model for fractionation

Hardi Peter and Eckart Marsch

Max-Planck-Institut für Aeronomie, D-37189 Katlenburg-Lindau, Germany (hpeter@linmpi.mpg.de)

Received 11 August 1997 / Accepted 3 February 1998

Abstract

A multi-fluid model for a hydrogen-helium mixture in an ionization-diffusion layer in the (solar) chromosphere is presented. The purpose of this model is to serve as a background for fractionation models calculating the abundance variations of minor species from the photosphere to the solar wind. The emphasis will be on the determination of the (mean) flow velocity in that ionization layer. The equations of continuity and momentum of every component, neutral and (singly) ionized for both elements, will be solved together with an energy equation including heating and radiative losses. Special attention will be paid to the ionization and the elastic collisions as well as to resonant charge exchange. One of the main results is the connection of the particle flux through the chromosphere with the ionization rate, i. e. with the photon flux in the UV. Furthermore the abundance variations of helium are discussed with the result, that the ion-neutral separation processes leading to the fractionation of the minor elements cannot explain the fractionation of helium as measured in the solar wind.

Key words: Sun: chromosphere Sun: abundances solar wind

Present address: High Altitude Observatory/NCAR, P.O. Box 3000, Boulder, CO 80307-3000, USA

Send offprint requests to: Hardi Peter

Contents

• 1. Introduction
• 2. Geometry and assumptions
  • 2.1. Geometry
  • 2.2. Assumptions
• 3. Model equations
  • 3.1. Multi-fluid transport equations
  • 3.2. Ionization, recombination and collisions
  • 3.3. Formulation of the transport equations appropriate for numerical treatment
  • 3.4. Boundary conditions
• 4. An analytical approximation for the hydrogen gas
• 5. Pure hydrogen gas: numerical studies
  • 5.1. Total density and ambipolar effects
  • 5.2. Total particle flux
  • 5.3. Typical chromospheric velocities
• 6. Inclusion of helium
  • 6.1. Diffusion
  • 6.2. Abundance and fractionation
  • 6.3. Helium abundance and origin of CMEs
• 7. Summary
• References

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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