PROPAGATION VELOCITY OF ALFVEN-WAVE PACKETS IN A DISSIPATIVE PLASMA

We have experimentally studied the behavior of Alfven wave packets in a dissipative plasma due to ion-neutral-atom collisions. It is urged t hat the central frequency of the packet is observed to gradually decre ase with traveling distance in the absorption range of frequencies bec ause of a differential damping among the Fourier components, and that the measured average velocity of its peak amplitude is not accounted f or by the conventional group velocity, but by the prediction derived b y Tanaka, Fujiwara, and Ikegami [Phys. Rev. A 34, 4851 (1986)]. Furthe rmore, when the initial central frequency is close to the critical fre quency in the anomalous dispersion, the wave packet apparently collaps es when traveling along the magnetic field; however, we have found tha t it is decomposed into another two wave packets with the central freq uencies being higher or lower than the critical frequency.