  The effects of second harmonics of the density and temperature perturbations
on the linear Geodesic Acoustic Mode (GAM) frequency and non-linear generation
of the GAM are investigated, using a fluid model. We show that the second
harmonics contribute to the frequency through the density gradient scale length
and the wave number of the GAM. In addition, the linear frequency of the GAM is
generally increased by coupling to the higher harmonic.
