On the Existence of Ionospheric Feedback Instability in the Earth's Magnetosphere‐Ionosphere System

The ionospheric feedback instability (IFI) has been considered one of the main generation mechanisms for large-amplitude ultralow frequency waves and small-scale field-aligned currents in the auroral and subauroral regions for more than 40 years. Sydorenko and Rankin (2017, https://doi.org/10.1002/2017GL073415) have recently challenged the very existence of the IFI for any realistic geophysical conditions in the Earth\u27s ionosphere-magnetosphere system. Because this conclusion contradicts numerous theoretical, numerical, and experimental works successfully used IFI to explain and predict results from observations for more than four decades, it deserves special attention. We show that this conclusion is mainly based on the specific ionospheric density profile and boundary conditions used in two runs of simulations presented in Sydorenko and Rankin (2017), and the generalization of this result is not justified. The effect of the collisions between ionospheric ions and neutrals on the development of the instability has been well studied since 1981, and these studies demonstrate that it does not prevent the development of the instability. Furthermore, excellent agreement of the theoretical and numerical results with observations verify without doubt the IFI existence and significance in the Earth\u27s magnetosphere-ionosphere system

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

The paper presents results from the numerical study of the magnetosphere-ionosphere interactions driven by the large-scale electric field in the magnetically conjugate, high-latitude regions of northern and southern hemispheres. Simulations of the two-fluid MHD model demonstrate that these interactions can lead to a generation of a system of small-scale, intense field-aligned currents with a significant difference in size and amplitude between the upward and downward currents. In particular, in both hemispheres, the downward currents (where the electrons are flowing from the ionosphere) become more narrow and intense than the adjacent upward currents. At high latitudes, the field-aligned currents are closely related to the discrete auroral arcs. The fact that this mechanism produces very narrow and intense downward currents embedded into the broader upward current regions makes it relevant to the explanation of the “black” auroral arcs appearing as narrow, dark strips embedded in the broad luminous background