The anomalous intensities of helium lines in the quiet solar transition region

Previous studies using observations made at low spatial and spectral resolu tion showed that the resonance lines of He I and He II are anomalously stro ng in the quiet Sun when compared with other transition region lines formed at similar temperatures. Here, the higher spatial and spectral resolution provided by the Coronal Diagnostic Spectrometer (CDS) instrument on board t he Solar and Heliospheric Observatory (SOHO) is used to re-examine the beha viour of the He I and He II lines and other transition region lines, in qui et regions near Sun centre. Supergranulation cell boundaries and cell inter iors are examined separately, Near-simultaneous observations with the SUMER instrument provide information on the lower transition region and the elec tron pressure. While the lines of He I and He II have a common behaviour, a s do the other transition region lines, the behaviour of the helium lines r elative to the other transition region lines is significantly different. Th e emission measure distributions that account for all transition region lin es, except those of helium, fail to produce sufficient emission in the He I and He II resonance lines by around an order of magnitude, in both supergr anulation cell boundary and cell interior regions. The electron pressure ap pears to be higher in the cell interiors than in the average cell boundarie s, although the uncertainties are large. While the VAL-D model gives a clos er match to the He I 584.3-Angstrom line, it does not successfully reproduc e other transition region lines.