Observation of coronal loop torsional oscillation

We suggest that the global torsional oscillation of solar coronal loop may be observed by the periodical variation of a spectral line width. The amplitude of the variation must be maximal at the velocity antinodes and minimal at the nodes of the torsional oscillation. Then the spectroscopic observation as a time series at different heights above the active region at the solar limb may allow to determine the period and wavelength of global torsional oscillation and consequently the Alfv{\'e}n speed in corona. From the analysis of early observation (Egan & Schneeberger \cite{egan}) we suggest the value of coronal Alfv{\'e}n speed as $\sim 500 {\rm km}{\cdot}{\rm s}^{-1}$.