Behaviour of Hydrogen Lyman lines in a prominence region from SUMER and CDS

We present observations of a prominence, taken on 1998 February 20 in the framework of SOHO Joint Observing Program no. 63. The instruments involved were SUMER and the NIS Spectrograph of CDS. The SUMER spectral range includes the hydrogen Lyman series – starting from Ly-2 – down to the head of the Lyman continuum, while CDS observed a number of lines from T ∼ 10 K to T ∼ 2×106 K. For these observations, we were able to obtain a satisfactory determination of the pointing of the SUMER slit relative to CDS. We thus examined – and compared with information from CDS spectra – the main characteristics of the hydrogen Lyman series lines and of other strong lines in the SUMER spectral interval. We also studied the properties (depth, asymmetry) of the central reversal present in several or all of the Lyman lines in some regions of the prominence.