Abstract
Abstract. The ionospheric feedback instability (IFI), which involves feedback between ionospheric modifications and waves reflected off the magnetosphere, has up to this point been analyzed in terms of field line integrated (FLI) ionospheric quantities, that is, with the assumption that the ionospheric thickness can be ignored. In this work we test this assumption by solving the two-fluid equations for a representative ionospheric slab of finite thickness. We find that the results are for the most part incompatible with a description in terms of FLI quantities, and that their use can easily lead to an order of magnitude overestimation of the growth rate. This occurs because the first eigenmode, which is the one compatible with an FLI description, is cutoff above a certain frequency, leaving only higher order modes with wavelengths along B that are subsumed by the slab. Taking the results at face value, the parallel electric fields associated with the higher order modes are a possible contributor to electron heating and plasma structure in the E-region ionosphere.
Highlights
Charge separation in the ionosphere launches an Alfven wave into the magnetosphere that transmits energy along the lines of the geomagnetic field, and reflects back to the ionosphere, where if the phase of the reflected wave is correct the initial charge separation will be reinforced, such that there is positive feed back and an instability
In the present work we examine the assumption, made in all previous works, that the ionosphere can be reasonably approximated by a thin layer – we find that it cannot
We describe the calculation of this modified admittance for a realistic magnetosphere, which will include the Fregion ionosphere; since the F-region does not play an active role in the ionospheric feedback instability (IFI), it can be lumped into the passive load that is presented to the top of the E-region ionosphere
Summary
Charge separation in the ionosphere launches an Alfven wave into the magnetosphere that transmits energy along the lines of the geomagnetic field, and reflects back to the ionosphere, where if the phase of the reflected wave is correct the initial charge separation will be reinforced, such that there is positive feed back and an instability. In the present work we examine the assumption, made in all previous works, that the ionosphere can be reasonably approximated by a thin layer – we find that it cannot We do this by performing a realistic computation of the complex magnetospheric admittance (using transmission line theory), and applying it to give a boundary condition at the top of a finite thickness ionosphere (described by 2-D two-fluid equations perpendicular and parallel to the magnetic field), and solving the eigenvalue problem for the growth rates of the eigenmodes
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