Self‐consistent ionospheric plasma density modifications by field‐aligned currents: Steady state solutions

The magnetosphere and ionosphere are coupled by field‐aligned currents that remove or deposit E‐region electrons. Changes in electron number density modify ionospheric reflectivity, hence altering the magnetospheric current. Thus, self‐consistent solutions are nontrivial. In this paper, we present 1‐D steady states that self‐consistently model modifications of ionospheric plasma density by field‐aligned currents. These are used to investigate the width broadening and minimum plasma density of E‐region plasma density cavities and the origin of small‐scale features observed in downward current channels. A plasma density cavity forms and broadens if the maximum initial current density j ∥0 exceeds j c = αn e 2 he /(1 + 1/ β ), where α is the recombination coefficient, n e is the equilibrium E‐region number density in the absence of currents, h is the E‐region thickness, and β = is the initial ratio of Pedersen to magnetospheric Alfvén conductivities. If a plasma density cavity forms, its final width increases monotonically with = 2 B 0 / μ 0 V A αn e 2 he , where B 0 is the background magnetic field strength and V A is the magnetospheric Alfvén speed. The minimum E‐region number density, and the finest length scale present in the steady state, both scale as 1/ β . For typical ionospheric parameters and j ∥0 = 5 μ Am −2 , the fine scale is comparable to or less than 6 λ e for β ≳ 2, where λ e is the electron inertial length. This suggests that electron inertial effects may become significant and introduce small‐scale features, following the production of a single fine scale by depletion and broadening.