Feedback Interactions Between the Ionosphere and Magnetosphere at Middle Latitude

Observations show that magnetic pulsations with frequencies around 1 mHz are frequently detected simultaneously at different latitudes on the ground, in the inner magnetosphere, and in the solar wind. The coupling between oscillations in the dynamic pressure or magnetic field carried by the solar wind and the ULF waves detected on the ground at high latitudes has been suggested in several studies. We present results from a numerical study of ultra-low-frequency waves detected by the ground magnetometers at middle latitudes during substorm. We investigate the hypothesis that these waves are generated by the ionospheric feedback instability driven by the large-scale electric field in the ionosphere. This field is associated with the surface waves propagating along the ambient magnetic field on strong perpendicular gradient in the plasma density occurring in the equatorial magnetosphere. The gradient in the plasma density is associated with the plasmapause. The plasmapause moves to the middle latitude when the plasmasphere erodes during substorm. The energy from the external driver can be coupled to the large-scale surface Alfven waves traveling along the field lines into the ionosphere and generate small-scale intense ULF waves and field-aligned currents at middle latitude. The simulations of the two-fluid MHD model confirms this scenario, and the numerical results show a good quantitative agreement with the observations.