High-Velocity Flows in an Active Region Loop System Observed with the Coronal Diagnostic Spectrometer (CDS) on SOHO

Abstract

EUV spectra of coronal loops above an active region show clear evidence of strong dynamical activity. We present an example where the O v 629 Å line, formed at 240 000 K, is shifted from its reference position corresponding to line-of-sight velocities greater than 50 km s^-1 with the shift extending over a large fraction of a loop. The observations were made with the Coronal Diagnostic Spectrometer (CDS) on the Solar and Heliospheric observatory (SOHO), and are from active region NOAA 7981 on the east solar limb on 27 July 1996. An animation has been prepared showing the variation of the shift or flow velocity along the loop. This animation is to be found on the enclosed CD-ROM and gives a clear impression of the dynamical condition present in the loop. The appearance of the loop system in different lines formed over a range in temperature as well as the observed dynamics indicates that loops at different temperatures are not closely co-located. Finally, the results are discussed and related to mechanisms that may cause line shifts.