Kelvin-Helmholtz vortices frequently develop at the magnetopause boundary of planetary magnetospheres where there is a shear in the flow. The nonlinear development of vortices involves coalescence, where smaller vortices combine into larger vortices in an inverse cascade. Simulations also show that in the later stages, the vortex structures are unstable due to the Rayleigh-Taylor instability caused by density gradients, and the coalescing structures can fragment into small scales in a forward cascade. This study examines the nonlinear cross-scale coupling in MHD and hybrid simulations of Kelvin-Helmholtz instability and characterizes the cascade directionality using transfer entropy. We view the polar spectral densities of the simulations to identify the timescales and directionalities of the cascade dynamics. We then perform a windowed transfer entropy analysis of the spectral densities to show how the dynamics of the cascades evolve over time. For each simulation we analyzed, the transfer entropy consistently displayed the inverse and forward cascade interactions we expected from the polar spectral densities, while also highlighting properties of the interactions that were not otherwise apparent. This study provides evidence that nonlinear coupling across scales is important in the turbulent process and demonstrates the usefulness of transfer entropy as a method of analyzing such processes.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)