SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Inter-network Chromosphere

Abstract

On 12 March 1996 we obtained observations of the quiet Sun with the SUMER instrument. The observa- tions were sequences of 15-20 second 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. 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". At any time they occur over approx. 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 UV (lambda > 912 A) continua, and they are also seen in the intensities and velocities of chromospheric lines of C I, N I and O I. 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. Three minute intensity oscillations are occasionally seen in second spectra (C II 1335), but never in third spectra (C III and Si III). Third spectra and He I 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 & Stein 1994) 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.