SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Internetwork Chromosphere

On 12 March 1996 we obtained observations of the quiet Sun with the SUMERinstrument. The observa- tions were sequences of 15-20 second exposures ofultraviolet emission line profiles and of the neighboring continua. These datacontain signatures of the dynamics of the solar chromosphere that are uniquelyuseful because of wavelength coverage, moderate signal-to-noise ratios, andimage stability. The dominant observed phenomenon is an oscillatory behaviorthat is analogous to the 3 minute oscillations seen in Ca II lines. Theoscillations appear to be coherent over 3-8". At any time they occur overapprox. 50 % of the area studied, and they appear as large perturbations in theintensities of lines and continua. The oscillations are most clearly seen inintensity variations in the UV (lambda > 912 A) continua, and they are alsoseen in the intensities and velocities of chromospheric lines of C I, N I and OI. Intensity brightenings are accompanied by blueshifts of typically 5 kms$^{-1}$. Phase differences between continuum and line intensities alsoindicate the presence of upward propagating waves. Three minute intensityoscillations are occasionally seen in second spectra (C II 1335), but never inthird spectra (C III and Si III). Third spectra and He I 584 show oscillationsin velocity that are not simply related to the 3 minute oscillations. Thecontinuum intensity variations are consistent with recent simulations ofchromospheric dynamics (Carlsson & Stein 1994) while the line observationsindicate that important ingredients are missing at higher layers in thesimulations. The data show that time variations are crucial for ourunderstanding of the chromosphere itself and for the spectral features formedthere.Comment: 8 pages, 3 figs, AASTeX, Accepted for publication in APJ letter