A global substorm electrodynamic model and a global ionospheric model were coupled in order to study ionospheric dynamics during substorms, with the focus on small-scale substorm electrodynamic and plasma structures. The simulation results show that in the expansion phase, structured precipitation and channeled field-aligned currents quickly develop in the substorm onset region. The Hall and Pedersen conductance ratio in the region increases significantly, and the magnetospheric field-aligned currents are mainly closed by highly structured Hall currents. Correspondingly, the plasma in the ionosphere also undergoes significant changes during a substorm and is highly structured in both the horizontal and vertical directions. In the substorm onset region, there are spatially separated small-scale Ti and Te hot spots, downward ExB drifts, decreased total electron contents, and a lowered ionosphere. Also, there is a significant O+ → NO+ conversion, leading to a great increase of NO+ and a lowering of the O+ peak height. These small-scale electrodynamic and plasma structures are very important for more realistically simulating the ionospheric dynamics during substorms. These results not only help to elucidate the multiscale ionospheric responses to substorms but also provide a theoretical guidance and cautions for the interpretation of various substorm observational data.