  We performed three-dimensional magnetohydrodynamic simulations of shear
Alfven waves in a full field line system with magnetosphere-ionosphere coupling
and plasma non-uniformities. Feedback instability of the Alfven resonant modes
showed various nonlinear features under the field line cavities: i) a secondary
flow shear instability occurs at the magnetic equator, ii) trapping of the
ionospheric Alfven resonant modes facilitates deformation of field-aligned
current structures, and iii) hybrid Alfven resonant modes grow to cause
vortices and magnetic oscillations around the magnetic equator. Essential
features in the initial brightening of auroral arc at substorm onsets could be
explained by the dynamics of Alfven resonant modes, which are the nature of the
field line system responding to a background rapid change.
