ULF Waves Generated by Magnetosphere-Ionosphere Interactions Near the Plasmapause During Substorms

We present results from a numerical study demonstrating that the small-scale electromagnetic waves frequently observed by satellites in the equatorial magnetosphere in the vicinity of the plasmapause can be generated by the magnetosphere-ionosphere interactions carried by the ULF field-aligned currents. These currents are produced by the ionospheric feedback instability driven by the large-scale electric field in the ionosphere. This quasi-stationary field is generated in the equatorial magnetosphere via a short-circuiting of the earthbound-ejected hot plasma flows by the plasmasphere. The field is mapped equipotentially along the geomagnetic field into the ionosphere in both hemispheres. This concept has been used to explain observations of small-scale, two-dimensional electromagnetic waves on the top of the large-scale electric field detected by the RBSP-A satellite in the vicinity of the plasmapause on March 17, 2015. The observations have been modeled with a two-fluid MHD code, describing propagation of the ULF waves and field-aligned currents in the magnetosphere and interactions between these currents and the ionospheric plasma. In particular, the model includes effects of the active ionospheric feedback on structure and amplitude of the currents causing variation in the ionospheric density/conductivity.

The simulations demonstrate that for the magnitude and structure of the large-scale electric field observed during the 03/17/2015 event, the ionospheric feedback instability develops when the conductivity in at least one of the hemispheres is relatively low. The instability produces ULF waves with frequencies and perpendicular sizes matching the observations in good, quantitative details. In particular, numerical results match observations in the wave frequency, amplitude and the perpendicular wavelength. The simulations also show that the instability develop in the localized region in the ionosphere where the necessary conditions for it are satisfied and it reaches some dynamic steady-state which can last for more than 40 minutes. This last finding explains the fact that these waves are observed in the vicinity of the plasmapause quite frequently.