Dynamic behavior of the upper solar atmosphere:: SUMER/SOHO observations of hydrogen Lyman lines

We present first observations of the temporal evolution of hydrogen Lyman lines, made by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory. A time series of about 33 minutes was obtained on 1997 June 5. The entrance slit has crossed a quiet-Sun region of 115." 3 with two internetwork structures (cells) and the bright network regions. A data set of 59 spectra with 30 s exposure time was obtained, using the SUMER tracking system to compensate for the solar rotation. For our analysis, we have selected a Lyman continuum window around 907 Angstrom and the three Lyman lines Ly5, Ly9, and Ly15, which are formed at different depths in the upper chromosphere. In the cell interiors, we have detected significant periodic intensity variations with a Fourier transform power peak at 3.3-3.5 minutes, which is consistent with 3 minute internetwork oscillations. They seem to be associated with spatially unresolved "clusters" of grains. In the bright network regions, we detect slower oscillations of 6.9-7.6 minutes. These waves seem to propagate upward as we deduce from a phase shift between the three Lyman lines studied. The phase velocity was estimated to be roughly 3 km s(-1) in the network. Finally, we discuss the potential usefulness of the hydrogen Lyman lines for diagnostics of the temperature structure of the upper solar atmosphere. Our observations, in particular the fact that we see all Lyman lines in emission all of the time, put certain constraints on the temperature gradients above the region in which numerical simulations do predict a decrease of the mean kinetic temperature.