Fig. 2: Aeolian transport phase diagram for perfectly bidisperse sand (terrestrial conditions).

Here, the sand bed is idealized as consisting of two grain species with diameters d^(f) and d^(c). The coarse grains can be incrementally kicked forward by the fine grains, resulting in a creeping motion known as reptation, if the wind shear stress τ falls within the bimodal transport regime (green-shaded area). It is delimited by the saltation thresholds τ_t(d^(f)) and τ_t(d^(c)) (dashed and dotted lines) of the fine and coarse grains, respectively, and the coarse-grain reptation threshold τ_r(d^(c)) (solid line). The thresholds are calculated from a physical grain-scale model (Methods) for typical terrestrial conditions (kinematic viscosity ν_a ≈ 1.6 × 10⁻⁵ m² s⁻¹, atmospheric density ρ_a ≈ 1.2 kg m⁻³, grain density ρ_p ≈ 2650 kg m⁻³ and fine-grain size d^(f) ≈ 491 μm, corresponding to Galileo number Ga^(f) ≈ 100 and density ratio s ≈ 2200). The transport regimes map directly onto dynamical regimes of sand sorting and megaripple evolution, as summarized in Table 1. To generalize this framework to more realistic continuous GSDs, as measured in the field, d^(f) is equated to the coarsest saltating grain size, \(\max ({d}^{({{{{{{{\rm{f}}}}}}}})})\), at a given wind strength τ. Thereupon, the transport phase diagram collapses onto the thick green line where \({\tau }_{{{{{{{{\rm{t}}}}}}}}}(\max ({d}^{({{{{{{{\rm{f}}}}}}}})}))=\tau\).