Vortices at the magnetic equator generated by hybrid Alfven resonant   waves
  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. 
Yasutaka Hiraki
physics.space-ph