Drift effects on electromagnetic geodesic acoustic modes
A two fluid model with parallel viscosity is employed to derive the dispersion relation for electromagnetic geodesic acoustic modes (GAMs) in the presence of drift (diamagnetic) effects. Concerning the influence of the electron dynamics on the high frequency GAM, it is shown that the frequency of the electromagnetic GAM is independent of the equilibrium parallel current but, in contrast with purely electrostatic GAMs, significantly depends on the electron temperature gradient. The electromagnetic GAM may explain the discrepancy between the $f\sim 40$ kHz oscillation observed in TCABR [Yu. K. Kuznetsov $\textit{et al.}$, Nucl. Fusion $\bf{52}$, 063044 (2012)] and the former prediction for the electrostatic GAM frequency. The radial wave length associated with this oscillation, estimated presently from this analytical model, is $λ_r\sim 25$ cm, i. e., an order of magnitude higher than the usual value for zonal flows (ZFs).