Ionospheric Alfvén resonator revisited: Feedback instability

Abstract

The theory of ionospheric Alfvén resonator (IAR) and IAR feedback instability is reconsidered. Using a simplified model of the topside ionosphere, we have reanalyzed the physical properties of the IAR interaction with magnetospheric convective flow. It is found that in the absence of the convective flow the IAR eigenmodes exhibit a strong damping due to the leakage of the wave energy through the resonator upper wall and Joule dissipation in the conductive ionosphere. It is found that maximum of the dissipation rate appears when the ionospheric conductivity approaches the “IAR wave conductivity” and becomes infinite. However, the presence of Hall dispersion, associated with the coupling of Alfvén wave modes with the compressional perturbations, reduces the infinite damping of the IAR eigenmodes in this region and makes it dependent on the wavelength. The increase in the convection electric field leads to a substantial modification of the IAR eigenmode frequencies and to reduction of the eigenmode damping rates. For a given perpendicular wavelength the position of maximum damping rate shifts to the region with lower ionospheric conductivity. When the convection electric field approaches a certain critical value, the resonator becomes unstable. This results in the IAR feedback instability. A new type of the IAR feedback instability with the lowest threshold value of convection velocity is found. The physical mechanism of this instability is similar to the Čerenkov radiation in collisionless plasmas. The favorable conditions for the instability onset are realized when the ionospheric conductivity is low, i.e., for the nighttime conditions. We found that the lowest value of the marginal electric field which is capable to trigger the feedback instability turns out to be nearly twice smaller than that predicted by the previous analysis. This effect may result in the decrease of the critical value of the electric field of the magnetospheric convection that is necessary for the formation of the turbulent Alfvén boundary layer and appearance of the anomalous conductivity in the IAR region.