Hydrogen temperature gradient in the transition region of a solar coronal hole

The Lyman series of hydrogen was observed by SUMER on SOHO on the north polar limb of the Sun with a total exposure time of more than ten hours. The resulting line profiles have been analysed using the technique described by Marsch et al. (1999). The data analysis corroborates earlier findings on the Lyman lines, but also yields phenomena which cannot be fully understood at the present time. Firstly, the line width of the Lyman lines increases with decreasing series or quantum number. Secondly, the hydrogen temperature gradient in the height range from 12 000 km to 18 000 km is unexpectedly small and does not reveal a steep jump as might be expected from modelling of the transition region. The average temperature increases only slightly from 1 105 K to 2 105 K. Possible explanations of these observations are given and models are briefly discussed.