SUMER observations confirm the dynamic nature of the quiet solar outer atmosphere: The internetwork chromosphere

On 1996 March 12, during the commissioning phase of the SOHO mission, we obtained observations of the quiet-Sun with the SUMER instrument. The observations were sequences of 15-20 s exposures of ultraviolet emission-line profiles and of the neighboring continua. These data contain signatures of the dynamics of the solar chromosphere that are uniquely useful because of wavelength coverage, moderate signal-to-noise ratios, and image stability. We focus on data for the internetwork chromosphere. The dominant observed phenomenon is an oscillatory behavior that is analogous to the 3 minute oscillations seen in Ca II lines. The oscillations appear to be coherent over 3''-8'' diameter areas. At any time they occur over about 50% of the area studied, and they appear as large perturbations in the intensities of lines and continua. The oscillations are most clearly seen in intensity variations in the ultraviolet (lambda > 912 Angstrom) continua, and they are also seen in the intensities and velocities of chromospheric lines of CI, NI, and OI. Intensity brightenings are accompanied by blueshifts of typically 5 km s(-1). Phase differences between continuum and line intensities also indicate the presence of upward propagating waves. The detailed behavior is different between different lines, sometimes showing phase lags. The 3 minute intensity oscillations are occasionally seen in second spectra (CII lambda 1335) but never in third spectra (C III and Si III). Third spectra and He I lambda 584 show oscillations in velocity that are not simply related to the 3 minute oscillations. The continuum intensity variations are consistent with recent simulations of chromospheric dynamics (Carlsson and Stein), while the line observations indicate that important ingredients are missing at higher layers in the simulations. The data show that time variations are crucial for our understanding of the chromosphere itself and for the spectral features formed there-the quiet-Sun's chromosphere is very dynamic and not ''quiet.'' The implications of these data should be considered when planning chromospheric work with instruments such as those on SOHO.