Structure and dynamics of an active region loop system observed on the solar disc with SUMER on SOHO

In this paper we present and discuss spectra obtained by SUMER on SOHO from an active region loop system observed on the solar disc, close to the central meridian, on July 26th, 1996. The region was observed with a spatial resolution of about 2′′ by 2′′ in emission lines forming in the transition region and inner corona, with the aim of investigating the physical structure and dynamical behaviour of the plasma in active region magnetic loops. To this purpose we have reduced and analysed the spectral observations in order to determine the values of intensity, Doppler shift and line profile width for the selected emission lines in all the spatially resolved elements of the examined area of the solar disc. By comparing intensity, velocity maps and photospheric magnetic fields obtained by MDI on SOHO, several magnetic loops have been identified, some of which contemporarily appear over a range of temperatures, while others are compact and only visible in a limited temperature range. A few loops exhibit velocity fields typical of siphon flows, the siphon-like velocities being higher in compact loops. Two compact loops seen in the transition region lines show asymmetric siphon-like velocity fields and high non-thermal velocities at the upflowing footpoint. High non-thermal velocities are also associated with the falling footpoint of a larger loop. Besides such loops, other bright features are observed in the transition region lines, whose morphology cannot be identified as arch-like. They have no coronal counterpart, are red-shifted with respect to the median line centroid position and exhibit high non-thermal velocities.