Modeling Minor Ion Abundances in Quiescent Coronal Loops

Abstract

We model minor element abundances in quiescent coronal loops anchored in the chromosphere, solving transport equations that give an improved description of thermal forces. Ionization, recombination, and radiative loss are included. The plasma consists of hydrogen and a minor element (oxygen, silicon, or iron), and we solve for all relevant charge states, including neutrals. We investigate the influence of the chromospheric composition and of varying the heating location on the loop abundance. If the chromosphere is well mixed, oxygen flows slowly into the loop due to the strong thermal force in the transition region, the maximum abundance exceeding the photospheric abundance by a factor of 4 after about one month. Silicon and iron are unable to flow past the area where hydrogen is ionized and therefore do not enter the loop. With a stratified chromosphere, the thermal force ensures that the minor ions present in the corona are stuck. Reasonable coronal abundances are then maintained if the loop contains photospheric abundances from the start. Heating near the footpoint causes minor ions to gather in the low corona, leaving the upper part severely depleted. Heat input high in the loop produces a thermal force that can counteract gravity and prevent settling, allowing high minor ion abundances at high altitudes, even for iron. Without fine tuning the model parameters, abundances vary by many orders of magnitude along the loop, and it is difficult to understand how the Sun maintains the much smaller variability inferred from observations.