Study of a transient siphon flow in a cold loop

Context: .The nature of loops is still a matter of debate with several explanations having been put forward. Simultaneous spectral and imaging data have the capacity to provide a new insight into mass motions, dynamics and energetics of loops.
Aims: .We report on spectral data taken with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) and imaging data from the Transition Region and Coronal Explorer (TRACE) of a transient event which occurred in a cold loop, lasting a few minutes.
Methods: .A sequence of TRACE images in the 1550 Å and 171 Å filters show a disturbance which originated at one foot-point and propagates along the loop. The SUMER slit was placed at the other foot-point of the loop. In order to interpret the results, numerical simulations were performed with the results then converted into observable quantities and compared with the data.
Results: .During the event a radiance increase and a relative red shift of ≈ 20~km s^-1 was detected in the N v 1238.82 Å line. 1D numerical simulations are performed and observable quantities derived from the results of the simulations. The observed dynamic behaviour of the N v 1238.82 Å line profiles was recovered.
Conclusions: .The results suggest that the observations could be interpreted in terms of a short-lived siphon flow reaching a speed of 120~km s^-1 and driven by a nonlinear heating pulse. The energies required to drive the observed red-shifts are estimated to be about 10²⁵ erg. The absence of a significant blue-shift caused by the return flow is explained.