
Abstract: Tertiary modes in electrostatic driftwave turbulence are localized near extrema of the zonal velocity $U(x)$ with respect to the radial coordinate $x$. We argue that these modes can be described as quantum harmonic oscillators with complex frequencies, so their spectrum can be readily calculated. The corresponding growth rate $\gamma_{\rm TI}$ is derived within the modified HasegawaWakatani model. We show that $\gamma_{\rm TI}$ equals the primaryinstability growth rate plus a term that depends on the local $U''$; hence, the instability threshold is shifted compared to that in homogeneous turbulence. This provides a generic explanation of the wellknown yet elusive Dimits shift, which we find explicitly in the TerryHorton limit. Linearly unstable tertiary modes either saturate due to the evolution of the zonal density or generate radially propagating structures when the shear $U'$ is sufficiently weakened by viscosity. The Dimits regime ends when such structures are generated continuously.
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1  README.txt  399 Bytes  
2  ARK_DATA.zip  1.34 MB 