Radial acceleration of geodesic acoustic modes in the presence of a temperature gradient

The global dynamics of geodesic acoustic modes (GAMs) is studied analytically and by means of gyrokinetic simulations for several equilibria with flat and nonuniform profiles. In particular, the effects of phase mixing via the continuum spectrum in the presence of a temperature gradient are investigated. We show that the frequency of GAM is not constant but can evolve in time because of the increase of the radial wavenumber. As a consequence, also the radial velocity of GAM increases in time. Thus, this study reduces the discrepancy between the linear theory and the experiments, in which strong velocities of GAM are generally observed. An estimate of phase and group velocity is given for simulations performed with experimental parameter values.