Unstable Eigenmodes of the Feedback Instability With Collision‐Induced Velocity Shear

A linear eigenmode analysis of the magnetosphere‐ionosphere coupling shows that the feedback instability leading to spontaneous formation of auroral arc structures remains unstable even in a case with strong vertical shear of horizontal ion flows induced by ion‐neutral collisions in the E layer. For low‐order Alfvén harmonics, the linear frequency and growth rate obtained by means of a height‐resolved model of the ionosphere are comparable to those resulted from the height‐integrated one, where fine vertical structures of the ionospheric density and magnetic field perturbations are well resolved in the former. The present result confirms validity of the height‐integrated ionosphere model in the long parallel wavelength limit, as assumed in previous studies on the feedback instability. Plain Language Summary Spontaneous formation of auroral arcs can be explained in terms of a plasma instability in the magnetosphere‐ionosphere coupling. The present study confirms validity and robustness of the basic instability theory even in cases with inhomogeneous profiles of the ionospheric parameters such as the ion‐neutral collision. Key Points Feedback instability in the magnetosphere‐ionosphere coupling is unstable even in a case with strong vertical flow shear in the E layer The linear eigenmode analysis captures entire profiles of the electromagnetic fields and the density perturbations along a field line A conventional magnetosphere‐ionosphere coupling model with a height‐integrated ionosphere remains valid in a low‐frequency regime