The stabilizing effect of collision‐induced velocity shear on the ionospheric feedback instability in Earth's magnetosphere

Abstract

The feedback instability in the ionospheric Alfvén resonator in Earth's magnetosphere is examined using a two‐dimensional multifluid numerical model of coupled ionosphere and magnetosphere. Two simulation configurations are used to demonstrate that the instability occurs under an assumption that is unrealistic for Earth's ionosphere. In the first configuration, a flat sheet height‐integrated conducting boundary replaces the ionospheric E layer. In the second configuration, plasma dynamics in a simplified E layer is resolved ignoring ion production, loss, and diffusion. For the same parameters (plasma and neutral density profiles and convection electric field), the instability develops only with the flat sheet boundary. When the E layer is resolved, the variation of ion‐neutral collision frequencies with altitude produces vertical shear in the horizontal ion flow velocity. The shear prevents density perturbations from remaining field aligned, causing them to decay rather than grow. It is suggested that the instability cannot occur in Earth's ionosphere because ion‐neutral collision frequencies always have a significant variation with altitude through the E layer.

Diagnosing the Role of Alfvén Waves in Magnetosphere‐Ionosphere Coupling: Swarm Observations of Large Amplitude Nonstationary Magnetic Perturbations During an Interval of Northward IMF

“…Understanding why downward FACs appear, at least during this northward IMF case study, to produce finer-scale (nonstationary) magnetic fluctuations is also a subject of future work. Theoretically, feedback should be favored by large-scale Journal of Geophysical Research: Space Physics 10.1002/2017JA024713 downward currents; however, recently, Sydorenko and Rankin (2017) have questioned the instability in a height-resolved ionosphere.…”

Ultralow Frequency Electrodynamics of Magnetosphere‐Ionosphere Interactions Near the Plasmapause During Substorms

“…For example, Sydorenko and Rankin () suggested that the inhomogeneity with the altitude of the ion‐neutral collision frequency can saturate IFI development or even prevent it from occurring because it creates a shear in the ion velocity at low altitudes which can “smooth out” the localized disturbances of the ionospheric conductivity which are an important part of IFI. However, this particular effect had been carefully investigated by Trakhtengertz and Feldstein (, , ), who demonstrated that this inhomogeneity can change the threshold and the growth rate of the instability, but it certainly does not prevent its occurrence when the magnitude of the electric field in the ionosphere exceeds the threshold value.…”

“…Inclusion of these effects will make the boundary conditions more complex and require more assumptions about various ionospheric parameters (because the direct measurements may not be conducted in the "right" place in the ionosphere when some events-of-interest are observed on satellites in the magnetosphere), but, what is very important, they do not change the basic physics of the ionospheric feedback interactions. For example, Sydorenko and Rankin (2017) suggested that the inhomogeneity with the altitude of the ion-neutral collision frequency can saturate IFI development or even prevent it from occurring because it creates a shear in the ion velocity at low altitudes which can "smooth out" the localized disturbances of the ionospheric conductivity which are an important part of IFI. However, this particular effect had been carefully investigated by Trakhtengertz and Feldstein (1981, 1984, who demonstrated that this inhomogeneity can change the threshold and the growth rate of the instability, but it certainly does not prevent its occurrence when the magnitude of the electric field in the ionosphere exceeds the threshold value.…”

On the Asymmetry Between Upward and Downward Field‐Aligned Currents Interacting With the Ionosphere

“…Recently, Sydorenko and Rankin () demonstrate with numerical simulations that under some particular conditions, the instability can be saturated by the inhomogeneity in the collision frequency between ions and neutrals in the ionospheric E region. This inhomogeneity causes shear in the ion velocity with altitude that can "smooth out" the localized density/conductivity disturbances in the ionosphere.…”