Ambiguity in Determining if Electromagnetic Perturbations Observed at Low Altitudes are Alfvénic or Quasi-static

There is significant interest within the space physics community in determining if electromagnetic perturbations are Alfvénic in nature. And further, determining if these perturbations are such that they can maintain an electric field parallel to the ambient background magnetic field, as this has important implications for particle acceleration and aurora. The usual approach to determining the nature of the perturbations is to assess the ratio of the electric to magnetic field. If this ratio is given by the local Alfvén speed, then the perturbations are assumed to be Alfvénic. In general, however, the ionospheric height-integrated Pedersen conductivity does not match the equivalent Alfvén wave conductivity (1/μ₀V_A where V_A is the Alfvén speed), and the waves are partially reflected. Immediately above the ionosphere the electromagnetic fields are given by the sum of the incident and reflected waves. Based on Snell’s law the electric to magnetic field ratio is consequently given by 1/μ₀Σ_P, where Σ_P is the height-integrated Pedersen conductivity. As a rough approximation the “near-field” region, where the field ratio is dependent on the Pedersen conductivity, is quarter of a wavelength. Because of this, low frequency Alfvén waves may instead be identified as quasi-static fields. Data from the low altitude TRACERS spacecraft will be used to demonstrate the frequency dependence of the transition from apparently quasi-static structures to Alfvénic.