Cross-scale energy transfer in the feedback magnetosphere-ionosphere coupling

In the high-latitude auroral region, the magnetosphere (M) and the ionosphere (I) are coupled through the Alfven wave propagation carrying the field-aligned current. The ionospheric current closure also leads to Joule heating of neutrals in the thermosphere (T). The Poynting flux associated with the magnetospheric convection is one of the major energy input in the M-I-T coupling. We have investigated the feedback instability in the auroral M-I coupling system, where the large-scale convection electric field drives growth of small-scale structures of electromagnetic fields and ionospheric density. It is found that the feedback instability growth can be regarded as an energy transfer from the convection scale to the auroral arc scale. Once the auroral structure has grown to a large amplitude, the sheared ExB flow drives the secondary instability, leading to smaller scale vortex structures. The nonlinear process in the feedback instability causes the energy transfer to finer spatial scales, and results in transition to the Alfvenic turbulence. The energy cascade in the turbulence continues to the dissipation scale where the viscous and Joule heating dominates. Our recent simulations of the feedback M-I coupling reveals the cross-scale energy transfer process, where a continuum energy spectrum appears in the magnetosphere. Interestingly, it is found that the vorticity spectrum shows a significant enhancement in a small scale on the ionosphere, while the corresponding power spectrum of the field-aligned current perturbations shows dominance of large scale components. Comparison of the present simulation results with observations should deepen our understandings on the cross-scale energy transfer in the M-I-T coupling.