Carole Jordan

Abstract:

As part of our study of the formation of the lines of He I and He II, observations of these lines and of other transition region and coronal lines have been made with the CDS on SOHO using hi-rate telemetry These give repeated 25-s exposures of the quiet Sun allowing time-dependent studies which complement our previous studies of the spatial variation of the He I, He II and other transition region lines. Network brightenings are observed which are similar to those previously reported by others. The variations of He I and He II lines with time have a smaller amplitude than those of the other transition region lines. This could be due in part to the higher opacity of the helium lines. Lines shifts and linewidths have also been investigated; some of our results confirm those found earlier, but many of the large downflows in the supergranulation cell boundaries are adjacent to, rather than co-spatial with, the highest intensities. An order of magnitude brightening, followed by a strong upflow and large turbulent velocities is also observed, and may be a small 'network flare'.

Abstract:

Previous studies (Jordan 1975, 1980) have found that the resonance lines of He I and He II appear to have higher fluxes than expected from models made using other transition region lines formed at similar temperatures. The enhancement factor was shown to be less in coronal holes than in the quiet Sun. These results referred to observations made with low spatial resolution. Using the higher spatial resolution provided by the CDS instrument onboard SOHO, we have started a systematic re-examination of the behaviour of the He I and He II line fluxes by examining several quiet Sun regions, from Sun-centre to near the limb, including a coronal hole. While the helium line fluxes are overall relatively lower in the coronal hole, their behaviour in the quiet Sun differs between the supergranulation cell boundaries and cell interiors. New simultaneous observations with the SUMER instrument are being used to investigate possible correlations between the electron density, non-thermal velocities and the helium line fluxes. We comment in passing on tests of the accuracy of the calibration of the CDS/NIS2 spectra.