A new interpretation of the ionospheric feedback instability applied to feld line resonances and the ionospheric Alfven resonator

It is demonstrated that the application of a height-integrated conductivity (HIC) boundary condition in theories of the ionospheric feedback instability is valid only for very thin (few km) conducting layers. In the presence of global convection, the strong variation of the ion mobility with altitude produces strongly sheared transverse ion flows within the E-layer.  These flows are not accounted for when the HIC boundary condition is applied, and when accounted for they cause a drastic reduction in growth rates of the IFI even for very large convection electric fields on the order of a few hundred mV/m. Thie reduction in IFI growth rates is verified through linear eigenmode analysis of the IFI similar to Watanabe & Maeyama (JGR, 45, 2018), except that (a) parallel electric fields in the ionosphere are accounted for, and (b) collision frequency profiles are determined from the IRI and MSIS models (Sydorenko and Rankin, GRL, 44, 2017).

The IFI in field line resonances (FLRs) and the ionospheric resonator (IAR) is studied for a collisional slab ionosphere of thickness 300 km. Constant density is assumed for FLRs, with the slab adjoining a collisionless plasma embedded in a constant magnetic field. Symmetry boundary conditions are applied at the equatorial magnetosphere. In the IAR study, the density varies with altitude and reflecting boundary conditions are used. Instability growth rates are computed numerically and compared with results for slabs of varying thickness (2 km to 300 km) and identical height-integrated conductivity. Growth rates for the most unstable mode are significantly reduced compared to the HIC case for layers as thin as 2 km, even in the long parallel wavelength limit.

The parallel electric field obtained from Faraday’s Law is strongly stabilizing for short transverse wavelength perturbations, especially for higher harmonics. A new unstable mode is found that does not require reflection of  waves within the IAR. It satisfies the resonance condition ω=k_y<V_d> where k_y is the transverse wavelength and <V_d> is the average ion drift velocity within the sptaially structured E-layer. The physical implication of this newly identified ionospheric instability is considered in the context of discrete auroral arcs and field line resonances.