Origin of the weakening of EUV emission lines formed in the chromosphere-corona transition zone

EUV spectroheliograms of the quiet Sun obtained with the Harvard experiment on Skylab are analyzed to identify the structure causing the weakening of the EUV line emission due to Lyman continuum absorption. The weakening at the network boundaries can be explained by overlapping of several spicules each of which being wrapped in an EUV emitting sheath. Part of the cell interiors show moderate weakening, this has the shape of a belt surrounding the network boundaries. There are a number of patches showing intense weakening near network boundaries and in cell interiors; the weakening at the points cannot be explained by overlapping of chromospheric structures with an EUV emitting sheath. A possible explanation is that the intense weakening is caused by cool chromospheric clouds or moving blobs over the EUV emitting sources in the cell interiors. Some of the points showing intense weakening are associated with an enhancement of the EUV emission. These points have lifetime shorter than the time interval of 5.5 min between successive observations, which stresses again that the chromosphere-corona transition zone is in a dynamic state.