Prominence-corona transition region plasma diagnostics from SOHO observations

New results concerning prominence observations and in particular the prominence - corona transition region (PCTR) are presented. In order to cover a temperature range from 2 x 10(4) to 7 x 10(5) K, several emission lines in many different ionization states were observed with SUMER and CDS on board SOHO. EM and DEM were measured through the whole PCTR. We compared the prominence DEM with the DEM from other solar structures (active region, coronal hole and the chromosphere - corona transition region (CCTR)). We notice a displacement of the prominence DEM minimum towards lower temperatures with respect to the minimum of the other structures. Electron density and pressure diagnostics have been made from the observed C III lines. Local electron density and pressure for T similar to 7 x 10(4) K are respectively log N-e = 9.30(-0.34)(+ 0.30) and 0.0405(-0.014)(+ 0.012). Extrapolations over the entire PCTR temperature range are in good agreement with previous SOHO results (Madjarska et al., 1999). We also provide values of electron density and pressure in two different regions of the prominence (center and edge). The Doppler velocity in the PCTR shows a trend to increase with temperature (at least up to 30 km s(-1) at T similar to 7 x 10(4) K), an indication of important mass flows. A simple morphological model is proposed from density and motion diagnostics. If the prominence is taken as a magnetic flux tube, one can derive an opening of the field lines with increasing temperature. If the prominence is represented as a collection of threads, their number increases with temperature from 20 to 800. Derived filling factors can reach values as low as 10(-3) for a layer thickness of the order of 5000 km. The variation of non-thermal velocities is determined for the first time, in the temperature range from 2 x 10(4) to 7 x 10(5) K. The quite clear similarity with the CCTR non-thermal velocities would indicate that heating mechanisms in the PCTR could be the same as in the CCTR (wave propagation, turbulence MHD).