THE HELIUM ABUNDANCE OF QUIESCENT CORONAL LOOPS

摘要

Coronal loops are modeled using a code that solves a newly developed set of transport equations, in which a more accurate description of thermal forces and heat conduction is contained. The plasma consists of hydrogen and helium, and ionization and recombination, as well as radiative loss, have been accounted for. When the loop is anchored in a "well-mixed" chromosphere with a helium abundance of approximately 10%, the helium density of the coronal part of the loop becomes larger than the hydrogen density within 1-3 days. In steady state, the loop is completely dominated by helium, with a helium density more than 10 times the hydrogen density. For a loop based in a stratified chromosphere with very little helium in the upper part, a moderate coronal helium abundance can be maintained. In this latter case, helium does not "drain" from the loop, and we get an extreme increase in the helium abundance from the upper chromosphere to the corona. These results suggest that the chromosphere underlying closed field regions of the Sun is highly stratified and that both helium and minor ion abundances are far from constant throughout the chromosphere.