Solitary zonal structures in subcritical drift waves: a minimum model

TL;DR

This paper demonstrates that solitary zonal structures in subcritical drift-wave turbulence can be modeled using a simplified reduced model, revealing their fundamental properties and a universal relation with zonal flows.

Contribution

The study introduces a minimal reduced model that captures the formation and properties of solitary zonal structures in subcritical drift waves, extending understanding beyond complex gyrokinetic simulations.

Findings

Solitary zonal structures can be reproduced in a reduced model with a modified Hasegawa-Mima equation.

These structures approximately satisfy a universal 'equation of state' linking drift wave envelopes and zonal flow velocity.

The reduced model serves as a minimal framework for understanding solitary zonal structures in subcritical drift-wave turbulence.

Abstract

Solitary zonal structures have recently been identified in gyrokinetic simulations of subcritical drift-wave (DW) turbulence with background shear flows. However, the nature of these structures has not been fully understood yet. Here, we show that similar structures can be obtained within a reduced model, which complements the modified Hasegawa-Mima equation with a generic primary instability and a background shear flow. We also find that these structures can be qualitatively reproduced in the modified Hasegawa-Wakatani equation, which subsumes the reduced model as a limit. In particular, we illustrate that in both cases, the solitary zonal structures approximately satisfy the same ''equation of state'', which is a local relation connecting the DW envelope with the zonal-flow velocity. Due to this generality, our reduced model can be considered as a minimum model for solitary zonal structures in subcritical DWs.