Geodesic acoustic modes (GAM) are oscillating zonal structures unique totoroidal plasmas, and have been extensively studied in the past decades due totheir potential capabilities of regulating microscopic turbulences andassociated anomalous transport. This article reviews linear and nonlineartheories of GAM; with emphases on kinetic treatment, system nonuniformity andrealistic magnetic geometry, in order to reflect the realistic experimentalconditions. Specifically, in the linear physics, the resonant wave-particleinteractions are discussed, with the application to resonant excitation byenergetic particles (EPs). The theory of EP-induced GAM (EGAM) is applied torealistic devices for the interpretation of experimental observations, andglobal effects due to coupling to GAM continuum are also discussed. Meanwhile,in the nonlinear physics, the spontaneous GAM excitation by microscaleturbulences is reviewed, including the effects of various systemnonuniformities. A unified theoretical framework of GAM/EGAM is thenconstructed based on our present understandings. The first-principle-basedGAM/EGAM theories reviewed here, thus, provide the tools needed for theunderstanding and interpretation of experimental/numerical results.
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