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We investigate properties of the ion-scale spectral break of solar windturbulence by means of two-dimensional high-resolution hybrid particle-in-cellsimulations. We impose an initial ambient magnetic field perpendicular to thesimulation box and add a spectrum of in-plane, large-scale, magnetic andkinetic fluctuations. We perform a set of simulations with different values ofthe plasma beta, distributed over three orders of magnitude, from 0.01 to 10.In all the cases, once turbulence is fully developed, we observe a power-lawspectrum of the fluctuating magnetic field on large scales (in the inertialrange) with a spectral index close to -5/3, while in the sub-ion range weobserve another power-law spectrum with a spectral index systematically varyingwith $\beta$ (from around -3.6 for small values to around -2.9 for large ones).The two ranges are separated by a spectral break around ion scales. The lengthscale at which this transition occurs is found to be proportional to the ioninertial length, $d_i$, for $\beta \ll 1$ and to the ion gyroradius, $\rho_i =d_i\sqrt{\beta}$, for $\beta \gg 1$, i.e., to the larger between the two scalesin both the extreme regimes. For intermediate cases, i.e., $\beta \sim 1$, acombination of the two scales is involved. We infer an empiric relation for thedependency of the spectral break on $\beta$ that provides a good fit over thewhole range of values. We compare our results with in situ observations in thesolar wind and suggest possible explanations for such a behavior.

Plasma beta dependence of the ion-scale spectral break of solar wind turbulence: high-resolution 2D hybrid simulations