The Ly-α profile in polar coronal holes

Polar regions often host extended areas, which appear dark in contrast to the hot coronal emission of the disk. Scientists named them coronal holes. There is little doubt that these are the source regions of the fast solar wind.

We show X-ray images of two southern coronal holes. In the white box the separation of the peaks of the Ly-α profile was measured: in the coronal hole, the separation (shown in black and purple) increases significantly, a clear indication of a larger opacity.

In the coronal hole the opacity is so high that the Ly-β line (which in contrast to Ly-α - as shown before - normally has a red peak dominance) now has a slight blue peak dominance and starts to behave like Ly-α.