Nonlinear effects in the ionospheric Alfvén resonator

Abstract

A two-dimensional nonlinear multi-fluid MHD model of the ionosperic Alfvén resonator is presented. The resonator is excited by a packet of shear Alfvén waves propagating downward toward the ionosphere from high altitudes. It is shown that the nonlinear (ponderomotive) force of standing oscillations in the ionospheric Alfvén resonator creates plasma flows along the geomagnetic field that modify the plasma density profile. Depending on the amplitude of the wave packet exciting the resonator, this effect can be very strong. For the selected plasma density profile in the near-Earth magnetosphere, an Alfvén wave packet with a smooth symmetric envelope and the maximum transverse electric field amplitude of 0.5 V/m, carrying 10 wave periods with the frequency of the first resonator harmonic, can decrease the plasma density by a factor of two at altitudes between 1000 and 1700 km in less than one minute. The amplitude of the wave packet is nonuniform in the direction across the geomagnetic field, which results in nonuniform density modification and phase mixing of oscillations on neighboring geomagnetic field lines. This increases both the transverse wavenumber of the excited resonator harmonics and the wave electric field along the geomagnetic field. Copyright 2008 by the American Geophysical Union.