Nonlinear Evolution of Auroral Fine Structures Through the Feedback Instability in a Dipole Field Configuration

Nonlinear simulations of the feedback instability in the Alfvénic magnetosphere‐ionosphere (M‐I) coupling system have been carried out by means of the reduced magnetohydrodynamic (MHD) equations with spatially non‐uniform kinematic viscosity and resistivity, coupled with two‐fluid equations, in order to investigate auroral fine‐structure formation in the dipole magnetic field. Spontaneous formation of vortex structures due to the secondary instability, followed by transition into turbulence, has been successfully reproduced. Collapse of the discrete auroral structures is accompanied by latitudinal expansion of turbulent fluctuations. The spatial scale and rotational sense of the vortex structures are consistent with those of auroral curls, and the energy spectrum of magnetospheric fluctuations in the turbulent state roughly follows the ‐law, and is consistent with the FAST spacecraft observation and the MHD turbulence theory.