SUMER Observations of the Quiet Solar Atmosphere: The Network Chromosphere and Lower Transition Region

Abstract

We examine spectral properties of the network chromosphere and lower transition region from the SUMER instrument on the SOHO spacecraft, using time-series data sets discussed in an accompanying Letter by Carlsson, Judge, & Wilhelm. The data were obtained early in the mission with no tracking of solar features and so cannot generally be used to examine intrinsic variations in features on timescales in excess of 383 s. Upon examination of the temporal variations and some preliminary power spectrum analysis, we find the following: (1) Transition region lines show more redshift in network regions than in internetwork regions and also a correlation between line intensity brightenings and increased redshift. (2) The internetwork "Ca II grain" phenomenon is not seen in He I λ584 or in lines of Si III and C III. (3) Very rapid changes are seen in the network for transition region lines with no obvious correspondence with the underlying chromosphere. (4) He I λ584 line profiles show very slow time variations. (5) Small-amplitude (2-5 km s−1) coherent oscillations of 5''-10'' scale length and ~130 s period are seen in Doppler shifts of Si III between regions of bright network elements. (6) Essentially all blueshifts or redshifts are substantially less than line widths. We conclude that upward-propagating acoustic shock waves do not contribute significantly to the heating of the lower transition region, and that ionization equilibrium is likely to fail for the interpretation of certain emission lines. The spatial coherency of the Si III velocity oscillations indicates that the quiet Sun's magnetic field topology is more uniform than emission-line intensity data alone might suggest.