Abstract. A new model of the ionospheric Alfv•n resonator (IAR) including the effect of wave frequency dispersion is presented. It is shown that the shear Alfv•n waves in the IAR are coupled to the compressional mode through the boundary conditions at the ionosphere. This coupling results in the appearance of the Hall dispersion and subsequent shift of the IAR frequency spectrum. The excitation mechanism involving the IAR interaction with the magnetospheric convective flow is considered. It is shown that the Hall dispersion of the IAR eigenmode increases the growth rate of the feedback instability. However, for the observed values of ionospheric conductivity this effect is not very high. It is shown that the physical mechanism of the feedback instability is similar to the Cerenkov radiation in collisionless plasmas. The IAR eigenfrequencies and growth rates are evaluated for the case of exponential variation of the Alfv•n velocity with altitude in the topside ionosphere. In this paper we will generalize the previous analysis by incorporating the dispersion of shear Alfvdn waves in the ionosphere-magnetosphere coupled system. We will show that this dispersion is produced by the ionospheric Hall current which arises due to the coupling of shear Alfvdn and fast magnetosonic (compressional) waves in the ionosphere. The deceleration of the Alfvdn phase velocity due to the Hall dispersion may increase the rate of energy transfer from the convective flow to IAR eigenmodes and overcome the dissipation rate due to the leakage of energy through the IAR upper boundary. The physical mechanism of such an instability, which we will term below as "feedback instability", is similar to the usual (•erenkov radiation in collisionless plasma.The paper is organized as follows: Section 2 describes the boundary conditions of the resonant cavity at the ionospheric and magnetospheric ends. The analysis of the dispersive ionospheric Alfvdn resonator is given in section 3. The dispersion relation for the feedback instability is presented in section 4. Excitation of dispersive eigenoscillations by the feedback instability in the lowconductivity ionosphere is considered in section 5. The case for a highly conductive ionosphere is analyzed in section 6. Our discussions and conclusions are found in section 7. 7737

Abstract. The theory of ionospheric Alfv6n 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 tha, t in the absence of the convective flow the IAR eigenmodes exhibit a strong da, mping due to the leakage of the wa,ve 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 Alfv•n 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 lea, ds to a. substantial modification of the IAR eigenmode frequencies a. nd to reduction of the eigenmode damping rates. For a given perpendicular wa,velength the position of inaximum damping rate shifts to the region with lower ionospheric conductivity. When the convection electric field approaches a certain critical va,lue, the resona,tor becomes unstable. This results in the IAR feedback instability. A new type of the IAR feedba, ck instability with the lowest threshold value of convection velocity is found. The physica,1 mecha, nism of this instability is similar to the Cerenkov radiation in collisionless plasma,s. The favorable conditions for the insta, bility 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 ca, pa, ble to trigger the feedback instability turns out to be nearly twice smaller tha, n tha,t predicted by the previous analysis. This effect may result in the decrease of the critical value of the electric field of the magnetospheric convection tha,t is necessary for the forma, tion of the turbulent Alfv•n boundary layer and appearance of the a, nomalous conductivity in the IAR region.