Alfv{é}nic Field Line Resonances in the Nightside Subauroral Zone

Results from a numerical study of localized, intense electromagnetic structures recently revealed from DMSP observations in the nightside sub-auroral zone are presented. It is shown that some of these structures can be interpreted as Alfv{é}n waves produced in a result of electromagnetic interaction between the equatorial magnetosphere and the nightside sub-auroral ionosphere. The main part of this interaction is the active ionospheric response (feedback) on a large-scale electric field generated in the equatorial magnetosphere and penetrating down to the ionospheric E-layer when the ionospheric Pedersen conductivity is low. In particular, computations show that when the Pedersen conductivity is less then 1 mho small-scale irregularities in the E-layer plasma density can generate Alfv{é}n waves with substantial amplitude. Initially, perpendicular electric and magnetic fields in those waves are in phase and corresponding Poynting vector is directed along the ambient magnetic field from the ionosphere. As time proceeds, the standing pattern of the resonant Alfv{é}n waves has been developing between the ionospheres and π /2 phase shift between wave E_{perpendicular to} and B_{perpendicular to} appears. In this time the Poynting vector can change several times direction along the ambient magnetic field within one structure. Both behaviors of Poynting vector were observed within intense electromagnetic structures registered by DMSP satellites in the sub-auroral zone. In this study the non-linear aspects of the ionosphere-magnetosphere coupling leading to the formation of small-scale intense electromagnetic structures are investigated depending on the parameters of the ionosphere and magnetospheric plasma along the entire sub-auroral flux tube. Computations show good quantitative agreement between numerical results and selected satellite and Millstone Hill radar observations.