A Computation on Velocity Field of the Post-flare Loop System of August 17, 1989

The observational line-of-sight velocity fields of a large post-f lare loop(PFL)system on August 17, 1989 were derived based on “multi-cloud model” (MCM)method from the Hβ spectral data obtained with the two-dimensional Multi-Waveband Spectra-Spectro Helio Graph (MWSSHG) at Yunnan Observatory.In order to explain the fundamental features of the velocity fields observed, an assumption and approximation to be taken into account that the matter within a loop system rises up from the loop foot to the loop top along the loop leg in a helical magnetic line under the combined action of solar gravity, magnetic stress and a gradient of atmospheric pressure in the loop system, and a theoretical velocity field of the loop system was calcuated with the aid of MHD theory.From a comparison of both velocity fields, it is found that the computed velocity field is basically similar with the observed one.It indicates that the assumption and approximation mentioned above are suitable basically.