  Evolution of a coronal loop in response to an impulsive energy release is
numerically modelled. It is shown that the loop density evolution curves
exhibit quasi-periodic perturbations with the periods given approximately by
the ratio of the loop length to the average sound speed, associated with the
second standing harmonics of an acoustic wave. The density perturbations have a
maximum near the loop apex. The corresponding field-aligned flows have a node
near the apex. We suggest that the quasi-periodic pulsations with periods in
the range 10-300 s, frequently observed in flaring coronal loops in the radio,
visible light and X-ray bands, may be produced by the second standing harmonic
of the acoustic mode.
