An Electromagnetic Calculation of Ionospheric Conductance that seems to Override the Field Line Integrated Conductivity

TL;DR

This paper develops a steady-state electromagnetic model for ionospheric conductance, revealing significant differences from traditional field-line-integrated conductivity across various scales, with implications for ionospheric and global Earth system modeling.

Contribution

It introduces a novel electromagnetic solution for ionospheric conductance that accounts for wave modes and mode-mixing, extending beyond electrostatic approximations.

Findings

Significant differences between electromagnetic and electrostatic conductance calculations.

Only two wave modes effectively transmit energy through the ionosphere.

The solution captures wavepacket and mode-mixing effects relevant to ionospheric science.

Abstract

We derive a steady-state, electromagnetic solution for collisional plasma and apply it to computing the total conductance for a vertically stratified ionosphere interrogated by a 3D signal with finite transverse wavelength, which we compare to the field-line-integrated conductivity, finding significant differences on all scales investigated. The approximate solution is derived from an exact linearization of the electromagnetic 5-moment fluid equations, and writing down the integral form of the driven steady-state solution, which is expressed as a sum over modal contributions associated with the eigenvectors/eigenvalues of the set of equations. We find the eigenvectors/eigenvalues numerically and use them to argue that only two of the modes are capable of transmitting energy through the ionosphere. Approximating their integrands with analytic functions yields a solution valid for homogeneous plasma, expressed in terms of the eigenvectors/eigenvalues for the two modes. The solution may be understood using wavepacket concepts. Transmission line theory is then used to extend the homogeneous-plasma solution to a solution for the vertically-stratified ionosphere. We review how transmission line theory contains electrostatic theory as a special case, and show that our solution reproduces electrostatic theory exactly when the eigenvectors/eigenvalues have the right properties, which requires that they be artificially modified. Otherwise we find short-wavelength, mode-mixing, and wave-admittance effects with broad relevance to ionospheric science, and to global modeling of the Sun-Earth system. Since resonant systems can be very sensitive, quantitative accuracy may require tuning, but the physical conclusions seem unavoidable and to affect all transverse scales.