On the emergence of a 1/k spectrum in the sub-inertial domain of turbulent media

Several physical and astrophysical fluid and plasma systems displaying turbulent features show a 1/k (or 1/f) spectral domain at spatial (temporal) scales above the inertial range. Here, we investigate the formation of a 1/k spectral domain in fluid and MHD turbulence systems in the sub-inertial range (k < k0, where k0 is the injection scale) by means of shell-model simulations. The simulation results suggest that the 1/k domain emerges as a consequence of competing direct and inverse cascading processes, implying the formation and dissipation of largescale gradients below the inertial domain. The relevance of our results for observations in space and astrophysical contexts is briefly discussed.