Linear mode conversion of Langmuir/z mode waves to radiation: Averaged energy conversion efficiencies, polarization, and applications to Earth's continuum radiation

Abstract

Linear mode conversion (LMC) is the linear transfer of energy from one wave mode to another in a density gradient. It is relevant to planetary continuum radiation, type II and III radio bursts, and ionospheric radio emissions. This paper analyzes LMC by calculating angle-averaged energy (ε) and power (εp) conversion efficiencies in both 2-D and 3-D for Langmuir/z mode waves (including upper hybrid waves for perpendicular wave vectors) converting to free-space radiation in turbulent plasmas. The averages are over the distributions of the incoming Langmuir/z mode wave vectors k, density scale lengths L, and angles α and δ, where α is the angle between k and the background magnetic field B0 and δ is the angle between the density gradient N0 and B0. The results show that the averaged and unaveraged conversion efficiencies are dependent on γβ, where γ is the adiabatic index and β is related to the electron temperature Te by β = Te/mec2. The averaged energy conversion efficiencies are proportional to γβ in 2-D and to (γβ)3/2 in 3-D, whereas the power conversion efficiencies are proportional to (γβ)1/2 in 2-D and γβ in 3-D. The special case of a perpendicular density gradient (δ≈90°) is considered and used to predict the conversion efficiencies of terrestrial continuum radiation (TCR) in three known source regions: the plasmapause, magnetopause, and the plasma sheet. The observed energy conversion efficiencies are estimated and are found to be consistent with the 2-D and 3-D predicted efficiencies; importantly, these results imply that LMC is a possible generation mechanism for TCR. The polarization of TCR is also predicted: TCR should be produced primarily in the o mode at the plasmapause and in both the o and x modes at the magnetopause and plasma sheet. These predictions are consistent with previous independent predictions and observations. Key Points Angle-averaged conversion efficiencies are derived for linear mode conversion Angle-averaged conversion efficiencies are orders of magnitude smaller than 1-D Predicted conversion efficiencies are consistent with estimates of observations ©2014. American Geophysical Union. All Rights Reserved.