Dispersive ionospheric Alfvén resonator

International audienceA 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

Ionospheric Alfv6n resonator revisited' Feedback instability I V. Khruschev i •/[. Parrot 2 S. Senchenkov, 1

International audienceThe 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

Linear theory of the mirror instability in non-Maxwellian space plasmas

A unified theory of the mirror instability in space plasmas is developed. In the standard quasi-hydrodynamic approach, the most general mirror-mode dispersion relation is derived and the growth rate of the mirror instability is obtained in terms of arbitrary electron and ion velocity distribution functions. Finite electron temperature effects and arbitrary electron temperature anisotropies are included. The new dispersion relation allows the treatment of more general space plasma equilibria such as the Dory-Guest-Harris (DGH) or Kennel- Ashour-Abdalla (KA) loss cone equilibria, as well as distributions with power law velocity dependence that are modeled by the family of kappa-distributions. Under these conditions, we derive the general kinetic mirror instability growth rate including finite electron temperature effects. As for an example of equilibrium particle distribution, we analyze a large class of kappa to suprathermal loss cone distributions in view of application to a variety of space plasmas like the solar wind, magnetosheath, ring current plasma, and the magnetospheres of other planets