Detection of mesogranulation at the upper chromosphere from SOHO/SUMER observations

Aims. Our aim is to detect the mesogranulation structures at the upper solar chromosphere Methods. We have analyzed a time series of spectra in the hydrogen Lyman lines and the Lyman continuum obtained by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on the SOlar Heliospheric Observatory (SOHO). The time series of about 2 hours and 22 minutes 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 Å, and the five Lyman lines: Ly5 (937.8 Å), Ly7 (926.2 Å), Ly9 (920.9 Å), Ly11 (918.1 Å), and Ly15 (915.3 Å). A Fast Fourier analysis has been performed in the spatial domain, all along the slit length used, for all the spectra and for the total duration of the observations. Results. We have detected a significant periodic spatial variations with Fourier transform power peaks around 9-10 arcsec and at 4 arcsec. They correspond to the scale of the mesogranulation structure and the width of the supergranular boundary, respectively. For the first time, this provides evidence for the existence of a meso-scale in the upper chromosphere, of the same size as observed in the photosphere and lower chromosphere by earlier studies. We find from the analysis that there seems to be no signature of any temporal evolution associated with the mesogranules, at least not during our observing period. This result suggests that the life time of mesogranules will be several hours or more, which confirms the earlier findings. In addition, we notice that the size (9-10 arcsec) of the mesocells appears to be the same in all Lyman lines and in the continuum, which are formed at different depths in the chromosphere.