Subauroral plasma convection during substorm cycle

The time evolution of the subauroral polarization stream (SAPS) during substorms is investigated using the HF radar observations at magnetic latitudes of 60-65 deg S. It is demonstrated that SAPS often exist for two or more substorm cycles. During the growth phase, the SAPS plasma convection is comparable to the background auroral convection. The subauroral plasma convection starts to intensify near substorm onset and peaks during the recovery phase. The response times to substorm onset range from -5 to +40 min and show some dependence on the substorm location with longer delays observed for substorms eastward of the radars' viewing area. It is proposed that the time evolution of the SAPS plasma convection is a two-stage process. During the initial phase, the plasma velocity is mostly controlled by the polarization electric field being established in the subauroral ionosphere after the injection of energetic ions. The time evolution following the initial increase may depend on both the ionospheric conditions through the ionosphere- magnetosphere coupling processes and additional supply of energetic ions during subsequent intensifications.