Active Regions Observed in Extreme Ultraviolet Light by the Coronal Diagnostic Spectrometer on SOHO

Abstract

We present observations of five active regions made by the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric observatory (SOHO). CDS observes the Sun in the extreme ultraviolet range 150–780 Å, Examples of active region loops seen in spectral lines emitted at various temperatures are shown. Several classes of loops are identified: those that are seen in all temperatures up to 2 × 10⁶ K; loops seen at 10⁶ K but not reaching 1.6 × 10⁶ K; those at temperatures 2–4 × 10⁵ K and occasionally at 6 × 10⁵ K but not reaching 10⁶ K. An increasing loop size with temperature and the relationship between the cool and hot structures is discussed. CDS observations reveal the existence of loops and other unresolved structures in active regions, at temperatures between 1.5– 4 × 10⁵ K, which do not have counterparts in lines emitted above 8 × 10⁵ K. Bright compact sources only seen in the transition region lines are investigated. These sources can have lifetimes of up to several days and are located in the vicinity of sunspots. We study the variability of active region sources on time scales from 30 sec to several days. We find oscillatory behaviour of He I and O V line intensities in an active region on time scales of 5–10 min.