Temporal and spatial variations of the quiet upper chromosphere from SOHO/SUMER observations of hydrogen Lyman lines

We have analyzed a time series of spectra in the hydrogen Lyman lines and t he Lyman continuum obtained by the Solar Ultraviolet Measurements of Emitte d Radiation (SUMER) spectrometer on the SOlar Heliospheric Observatory (SOH O). The time series of about 2 hours and 22 min was obtained on 1999 March 9 in a quiet region near the center of the solar disk. For our analysis, we have selected a Lyman continuum window around 907 Angstrom, and the five L yman lines: Ly5 (937.8 Angstrom), Ly7 (926.2 Angstrom), Ly9 (920.9 Angstrom ), Ly11 (918.1 Angstrom), and Ly15 (915.3 Angstrom). We derived the central intensity of these lines for a large number of locations over the network and inter-network regions. We found from their line-center intensity time s eries and from the power spectrum analysis that the network and inter-netwo rk regions at the upper chromosphere are associated with 5-7 min and 3-min period of intensity oscillations, respectively. Phase difference analysis s hows that there is a preponderance of upward-propagating waves in the upper chromosphere. The phase velocity was estimated to be roughly 4-5 km s(-1) in the network regions. In addition, a Fast Fourier analysis has been perfo rmed in the spatial domain, all along the slit length used, for all the spe ctra and for the total duration of the observations. We have detected signi ficant periodic spatial variations with Fourier transform power peaks aroun d 9-10 arcsec and at 4 arcsec. They correspond to the scale of mesogranulat ion structure and the width of the supergranular boundary, respectively. Th is provides evidence for the existence of a meso-scale in the upper chromos phere, of the same size as observed in the photosphere and lower chromosphe re by earlier studies. We notice that the size (9-10 arcsec) of the meso-sc ale structures appears to be the same in all Lyman lines and in the continu um, which are formed at different depths in the chromosphere.