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Linear mode conversion of Langmuir/ z mode waves to radiation: Averaged energy conversion efficiencies, polarization, and applications to Earth's continuum radiation
Author(s) -
Schleyer Fiona,
Cairns Iver H.,
Kim EunHwa
Publication year - 2014
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2013ja019364
Subject(s) - physics , computational physics , radiation , atomic physics , adiabatic process , plasma , polarization (electrochemistry) , langmuir probe , optics , plasma diagnostics , chemistry , quantum mechanics , thermodynamics
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 ( ε p ) 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 B 0 and δ is the angle between the density gradient ∇ N 0 and B 0 . The results show that the averaged and unaveraged conversion efficiencies are dependent on γ β , where γ is the adiabatic index and β is related to the electron temperature T e by β = T e / m e c 2 . The averaged energy conversion efficiencies are proportional to γ β in 2‐D and to ( γ β ) 3/2 in 3‐D, whereas the power conversion efficiencies are proportional to ( γ β ) 1/2 in 2‐D and γ β 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.