A significant part of the solar atmosphere consists of areas of closed magnetic field lines, and observations show that coronal loops come in a wide variety of sizes and can remain relatively unchanged for hours and days.
We model coronal loops that extend from the chromosphere and consist of hydrogen and helium. In the transition region and low corona the so-called ``thermal force'' can become important. This collisional force acts to push heavy ions, such as helium ions, upwards. The thermal force can be stronger than gravity and may therefore lead to large increases in helium abundances from the upper chromosphere to the corona. On the other hand, available observations place the coronal helium abundance in the 10 % range. In this presentation we show that reconciling the model results with coronal observations requires a highly stratified chromosphere with very little helium in the upper part.
We have also started to model oxygen in loops. Preliminary results indicate that the thermal force plays an even larger role for oxygen ions, causing huge changes in the oxygen abundance along the loop.