Active region loops observed with SUMER on board the Solar and Heliospheric Observatory

We study the emission and dynamical characteristics of transition region temperature plasmas in magnetic loops by analyzing a high-resolution, limb observation of the active region NOAA 7962. The observations were performed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the Solar and Heliospheric Observatory (SOHO). The SUMER observation produced a set of raster scans of the region, in the four lines, H I Lyβ λ1025, O VI λλ1032, 1038, and C II λ1037. The data are used to construct intensity, velocity, and line width maps of the active region, from which more than 10 well-resolved loops are identified and classified into four different groups. We determine several physical parameters of the loops in each group such as diameter, length, temperature, line-of-sight plasma velocity, and nonthermal line broadening. Our results indicate that both kinds of temperature variations exist in active region loops: variations from loop to loop and variations along each loop. It is also found that there is a distinction between stationary loops and dynamic loops. The dynamic loops have large bulk motions and large nonthermal line broadenings. Some of the dynamic loops display large velocity shears with the sign of line-of-sight velocities changing across the loop axes. These velocity shears appear to represent rotational motions around the loop axes with velocities of up to 50 km s-1. There are indications that nonthermal line broadening is the result of magnetohydrodynamic turbulence inside the loops. Based on our observations, we postulate that when loops erupt, some of the kinetic and magnetic energy cascades down to turbulent energy which would be dissipated as heat.