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 jc = αne2he/(1 + 1/β), where α is the recombination coefficient, ne 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 �� = 2B0/μ0VAαne2he, where B0 is the background magnetic field strength and VA 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 μAm−2, the fine scale is comparable to or less than 6λe for β ≳ 2, where λe 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.