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

Abstract

Previous studies using observations made at low spatial and spectral resolution showed that the resonance lines of Hei and Heii are anomalously strong 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 the Solar and Heliospheric Observatory (SOHO) is used to re-examine the behaviour of the Hei and Heii lines and other transition region lines, in quiet regions near Sun centre. Supergranulation cell boundaries and cell interiors are examined separately. Near-simultaneous observations with the sumer instrument provide information on the lower transition region and the electron pressure. While the lines of Hei and Heii have a common behaviour, as do the other transition region lines, the behaviour of the helium lines relative to the other transition region lines is significantly different. The emission measure distributions that account for all transition region lines, except those of helium, fail to produce sufficient emission in the Hei and Heii resonance lines by around an order of magnitude, in both supergranulation cell boundary and cell interior regions. The electron pressure appears to be higher in the cell interiors than in the average cell boundaries, although the uncertainties are large. While the VAL-D model gives a closer match to the Hei 584.3-Å line, it does not successfully reproduce other transition region lines.