Active ionospheric role in small-scale aurora structuring

The auroral structures on the spatial scales of 10s-100s km and their associated electrodynamics are key elements in the magnetosphere-ionosphere (M-I) system and are important for ionospheric dynamics. The simulation of these auroras poses a challenge for global magnetohydrodynamic (MHD) M-I models. In this work, an M-I coupling mechanism for the formation of auroral structures on these spatial scales is proposed. In the M-I coupling mechanism, the active role of the ionosphere is taken into account and the auroral structures with spatial scales of 10s-100s km, which do not mirror the features of the magnetospheric drivers, self-consistently develop. The proposed mechanism includes the effects of the ionospheric spatial and temporal scales and the M-I coupling processes. By calculating the reflection of the Alfvén waves at the ionosphere and the Alfvén waves launched by the temporal variation of the ionospheric conductivity, the mechanism can provide time-dependent, quantitative information of all electrodynamic parameters associated with the dynamic evolution of structured auroras, including the field-aligned currents, ionospheric horizontal currents, convection field, conductivity, and Joule heating rate. The M-I coupling mechanism has been quantitatively tested and validated by using numerical simulations and model-observation comparisons. Some of the theoretical predictions have been independently confirmed by observations. With its strengths of (a) quantitative capability and (b) emphasize on the active role of the ionosphere, this M-I coupling mechanism complements other mechanisms for the auroral structures as well as the global MHD M-I modeling, thus improving our understanding of the auroral structures and the M-I coupling processes.