As a result of shearing zonal flows, many plasma systems exhibit basically no transport until well above the linear marginal stability threshold. This effective upshift of the critical gradient is known as the Dimits shift, a detailed description of which is still lacking. To remedy this, in this work the role of the tertiary instability, within and beyond the Dimits shift regime of different collisionless systems, is investigated. Through tertiary linear stability analysis and nonlinear simulations, both of simple fluid systems and full gyorokinetics as simulated by GENE, it is found that in simple geometries the collisionless Dimits transition corresponds to a linear tertiary transition. An efficient reduced prediction model exploiting this fact is therefore postulated and finally verified.
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