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Magnetised plasma turbulence can have a multiscale character: instabilitiesdriven by mean temperature gradients drive turbulence at the disparate scalesof the ion and the electron gyroradii. Simulations of multiscale turbulence,using equations valid in the limit of infinite scale separation, reveal novelcross-scale interaction mechanisms in these plasmas. In the case that bothlong-wavelength (ion-gyroradius-scale) and short-wavelength(electron-gyroradius-scale) linear instabilities are driven far from marginalstability, we show that the short-wavelength instabilities are suppressed byinteractions with long-wavelength turbulence. The observed suppression is aresult of two effects: parallel-to-the-field-line shearing by the longwavelength $\mathbf{E} \times \mathbf{B}$ flows, and the modification of thebackground density gradient by long-wavelength fluctuations. In contrast,simulations of multiscale turbulence where instabilities at both scales aredriven near marginal stability demonstrate that when the long-wavelengthturbulence is sufficiently collisional and zonally dominated the effect ofcross-scale interaction can be parameterised solely in terms of the localmodifications to the mean density and temperature gradients. We discussphysical arguments that qualitatively explain how a change in equilibrium driveleads to the observed transition in the impact of the cross-scale interactions.

Stabilisation of short-wavelength instabilities by parallel-to-the-field shear in long-wavelength $\mathbf{E} \times \mathbf{B}$ flows