  There are two distinct phases in the evolution of drift wave envelope in the
presence of zonal flow. A long-lived standing wave phase, which we call the
Caviton, and a short-lived traveling wave phase (in radial direction) we call
the Instanton. For drift wave turbulence driven by ion temperature gradient
mode (ITG), these two stages of dynamics were displayed in [Zhang Y Z, Liu Z Y,
Xie T, Mahajan S M and Liu J 2017 Physics of Plasmas 24 122304]. In this paper
we show that the dynamical attributes of ITG turbulence are readily replicated
when the turbulence rotates in the electron direction; our model calculation
deals specifically with the toroidal electron drift waves (EDW) in the
well-known \delta_e model. While the basic calculations are presented in
parallel to the ITG counterpart, more emphasis is laid here on the motion of
Instanton; several abrupt phenomena observed in tokamaks, such as intermittent
excitation of geodesic acoustic mode (GAM) shown in this paper, could be
attributed to the sudden and fast radial motion of Instanton. The calculation
brings out the defining characteristics of the Instanton: it begins as a linear
traveling wave right after the transition. Then, it evolves to a nonlinear
stage with increasing frequency all the way to 20 kHz. The modulation to
Reynolds stress in zonal flow equation will cause resonant excitation to GAM.
The intermittency is shown due to the random phase mixing between multiple
central rational surfaces in the reaction region.
