Nonlinear effects in the ionospheric Alfvén resonator

Abstract

[1] 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.

Electron acceleration by an Alfvénic pulse propagating in an auroral plasma cavity

“…This confirms the results shown in previous works, conducted with only one sinusoidal wave [Génot et al, 2001a[Génot et al, , 2004. It is at odds with Sydorenko et al [2008] in which numerical simulations of an Alfvén wave packet of high-amplitude propagating in the ionospheric Alfvén resonator are conducted. Contrary to the present work, they initialized a wave with a transverse wave vector.…”

Non-propagating electric and density structures formed through non-linear interaction of Alfvén waves

“…In that case, the density cavities are not created at magnetospheric altitudes, but in the bottomside ionosphere, a region that can be observed with ground based radars. Still on a large scale, the ponderomotive force associated to standing oscillations in the ionospheric Alfvénic resonator can create plasma flows along the geomagnetic field that modify the plasma density profile (Sydorenko et al, 2008).…”

Simulation of O+ upflows created by electron precipitation and Alfvén waves in the ionosphere

“…[39] The ponderomotive effect in IAR was considered previously by Streltsov and Lotko [2008] and Sydorenko et al [2008]. Figure 5 of Streltsov and Lotko [2008] shows modification of ion density by the ponderomotive force.…”

“…The model of Sydorenko et al [2008] also considers collisionless plasma but it uses a perfectly conducting surface as the boundary condition at the low-altitude end. As a result of this simplified boundary condition, the electromagnetic field pattern in the simulation of Sydorenko et al [2008] is such that the ponderomotive force near the low-altitude boundary is directed upward [see Sydorenko et al, 2008, Figure 7]. Thus, the imposition of a perfectly conducting layer boundary condition has important consequences for the direction of the ponderomotive force near the boundary.…”