Dynamic features observed with SOHO-CDS

The transition region emission in the extreme ultraviolet (EUV) wavelength region is characterised by a bewildering array of dynamic events in the quiet Sun, coronal holes and active regions. In this paper we seek to explore the nature of these dynamic events using the CDS (Coronal Diagnostic Spectrometer) on SOHO (Solar Heliospheric Observatory). The CDS instrument provides an opportunity to study the EUV (151-785 Angstrom) wavelength range with good spectral, spatial and temporal resolution. Details of the CDS instrument and first science results are presented in Harrison et al (1997). In this review, we concentrate on results from the normal incidence spectrometer (NIS), which covers two wavelength ranges (307-379 and 513-633A). With the NIS, simultaneous observations can be obtained over a wide range of temperatures, so that the spatial relationship between the chromospheric, transition region and coronal emission can be unambiguously established. The CDS also allows velocity shifts to be determined from the spectral line profiles.