Variability and dynamic state of active region loops

A set of 218 consecutive CDS rasters taken at the solar limb on October 26–28 1999 has been used to investigate the variability and plasma dynamics of active region loops. Each raster contains simultaneous images in 6 different lines, covering the full temperature range of CDS, 10 000 K (He I) to 2.7 MK (Fe XVI). Activity is seen to go on without breaks at temperatures below 1 MK for the full 39 hours of the series. Transition region loops or extended sections of loops, 50–200 Mm long, appear and disappear in intervals as short as 11 minutes, the observing cadence. In the corona the emission is less variable, but significant changes are seen. Measured Doppler shifts correspond to typical plasma velocities of 20 km s −1 to 100 km s −1, at temperatures 10 000 K to 450 000 K, and siphon flows may occur in some of the loops. High velocities are frequently seen where the emitted intensities are weak, often on the outer edges of loops as defined in that particular spectral line. At coronal temperatures, 1 MK and higher, systematic loop velocities occur only occasionally. Simultaneous observations with EIT and SUMER were made during part of the raster series and are compared with the CDS result.