A simulation study of kilometric radiation generation along an auroral field line

Abstract

Relativistic modifications to wave dispersion near the electron gyrofrequency are known to be important in a low‐density, energetic plasma. Previous investigations based on simple model distribution functions have suggested that these relativistic dispersion effects may substantially alter the predictions of the cyclotron maser theory as applied to the generation of auroral kilometric radiation (AKR). The present work combines a model for the high‐altitude auroral zone with local particle simulations in order to assess the quantitative implications of relativistic dispersion for the generation of AKR. Electron distribution functions, which include a loss cone and a “hole” at lower energies produced by the parallel electric field, are determined along an auroral field line. Number densities of both energetic auroral electrons and low‐energy backscattered electrons are obtained from a fit to the observed density‐depleted region on an auroral field line. It is found that the hot magnetospheric component is the dominant species at altitudes greater than 1.7 RE geocentric. These auroral zone parameters and distributions are used as input to one‐ and two‐dimensional particle simulations. The simulations show that the electric field results in more free energy in the hot electron population at the lower altitudes. This is counteracted, however, by the increased density of backscattered and secondary electrons at these altitudes, which tends to quench the maser instability. The most intense radiation is found to occur between 1.75 and 2.0 RE, corresponding to AKR frequencies in the range 200‐300 kHz. The peak in the radiation is very close to 90° to the ambient magnetic field, with an offset of a few degrees in the direction of the loss cone. The dominant emission is in the extraordinary mode with a wave frequency ω/Ωe ≈ 0.99. A weak (∼1%) ordinary mode component is produced in association with the extraordinary mode due to a relativistic coupling between perpendicular wave electric fields and parallel currents.