  Through a systematically developed theory, we demonstrate that the motion of
instanton identified in [Y. Z. Zhang, Z. Y. Liu, T. Xie, S. M. Mahajan, and J.
Liu, Physics of Plasmas 24, 122304 (2017)] is highly correlated to the
intermittent excitation and propagation of geodesic acoustic mode (GAM) that
are observed in tokamaks. While many numerical simulations have observed the
phenomena, it is the first theory that reveals the physical mechanism behind
GAM intermittent excitation and propagation. The preceding work is based on the
micro-turbulence associated with toroidal ion temperature gradient (ITG) mode,
and slab-based phenomenological model of zonal flow. When full toroidal effect
are introduced into the system, two branches of zonal flow emerge: the
torus-modified low frequency zonal flow (TLFZF), and GAM, necessitating a
unified exploration of GAM and TLFZF. Indeed, we observe that the transition
(decay) from the caviton to instanton is triggered by a rapid zero-crossing of
radial group velocity of drift wave and is found to be strongly correlated with
the GAM onset. Many features peculiar to intermittent GAMs, observed in real
machines, are thus identified in the numerical experiment. The results will be
displayed in figures and in a movie; first for single central rational surface,
and then with coupled multiple central rational surfaces. The periodic bursting
first shown disappears as being replaced by irregular one, more similar to the
intermittent characteristics observed in GAM experiments.
