Mode Conversion and Reflection of Langmuir Waves in an Inhomogeneous Solar Wind

Willes, A. J.; Cairns, Iver H.

doi:10.1071/as01051

Cited by 30 publications

(45 citation statements)

References 14 publications

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“…The maximum values of ǫ are less than ≈ 8% for all q and Ω, with peaks of about 5% and 8% near (q, Ω) = (0.5, 0) and (0.3, 1.3), respectively. These are smaller than the values of 50 − 70% reported previously [16,17,[19][20][21]23]; however, these previous values were calculated from the power fluxes and so actually correspond to ǫ s . Using the same definition as previous authors, with…”

Section: A the Total Conversion Efficiency ǫ(Q ω)contrasting

confidence: 50%

“…Predictions for the maximum value of θ L in can be derived as follows, extending an earlier analysis [21]. The maximum values of q for mode conversion to the o and x modes are defined by sin are angles between B 0 and the outgoing o-and x-mode wavevectors.…”

Section: All Variables Vary Asmentioning

confidence: 99%

“…Early workers favored nonlinear mechanisms due to the very highly nonthermal levels of Langmuir waves involved and because calculations suggested that the efficiency of LMC was inadequate. However, subsequent calculations for 1-D linear density gradients yield (power) conversion efficiencies ǫ ≈ 50 − 70% in a narrow angular window dependent on the properties of incoming Langmuir/z waves and density structures [16][17][18][19][20][21][22][23]. It was found that in magnetized plasmas ǫ slightly decreases and the mode conversion window becomes narrower compared with unmagnetized calculations [16,19].…”

Section: Introductionmentioning

confidence: 99%

“…Willes and Cairns [21] generalized Eq. (9) to X 0 = 0 in the unmagnetized limit, finding that ǫ is primarily a function of…”

Section: All Variables Vary Asmentioning

confidence: 99%

“…They solved the full wave equations numerically using a fluid simulation code. The code studies Langmuir/z waves approaching a linear density gradient from a region with constant density, as well as the o, x, and Langmuir/z mode waves that leave this region, extending the approach of Willes and Cairns [21]. Many earlier analyses focused solely on calculating the total conversion efficiency [18-20, 22, 23], which may be determined without resorting to a full wave field solution, or introduced approximate analytic simplifications.…”

Section: Introductionmentioning

confidence: 99%

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Mode Conversion of Langmuir to Electromagnetic Waves with Parallel Inhomogeneity in the Solar Wind and the Corona

Kim¹,

Cairns²,

Robinson³

2008

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Linear mode conversion of Langmuir waves to radiation near the plasma frequency at density gradients is potentially relevant to multiple solar radio emissions, ionospheric radar experiments, laboratory plasma devices, and pulsars. Here we study mode conversion in warm magnetized plasmas using a numerical electron fluid simulation code with the density gradient parallel to the ambient magnetic field B0 for a range of incident Langmuir wavevectors. Our results include: (1) Both oand x-mode waves are produced for Ω ∝ (ωL) 1/3 (ωc/ω) 1, contrary to previous ideas. Only o mode is produced for Ω 1.5. Here ωc is the (angular) electron cyclotron frequency, ω the angular wave frequency, and L the length scale of the (linear) density gradient. (2) In the unmagnetized limit, equal amounts of o-and x-mode radiation are produced. (3) The mode conversion window narrows as Ω increases. (4) As Ω increases the total electromagnetic field changes from linear to circular polarization, with the o-and x-mode signals remaining circularly polarized. (5) The conversion efficiency to the x mode decreases monotonically as Ω increases while the o-mode conversion efficiency oscillates due to an interference phenomenon between incoming and reflected Langmuir/z modes. (6) The total conversion efficiency for wave energy from the Langmuir/z mode to radiation is typically less than 10%, but the corresponding power efficiencies differ by the ratio of the group speeds for each mode and are of order 50 − 70%. (7) The interference effect and the disappearance of the x mode at Ω 1 can be accounted for semiquantitatively using a WKB-like analysis. (8) Constraints on density turbulence are developed for the x mode to be generated and be able to propagate from the source. (9) Standard parameters for the corona and the solar wind near 1 AU suggest that linear mode conversion should produce both o-and x-mode radiation for solar and interplanetary radio bursts. It is therefore possible that linear mode conversion under these conditions might explain the weak total circular polarizations of type II and III solar radio bursts.

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Section: A the Total Conversion Efficiency ǫ(Q ω)contrasting

confidence: 50%

Section: All Variables Vary Asmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…Willes and Cairns [21] generalized Eq. (9) to X 0 = 0 in the unmagnetized limit, finding that ǫ is primarily a function of…”

Section: All Variables Vary Asmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Mode Conversion of Langmuir to Electromagnetic Waves with Parallel Inhomogeneity in the Solar Wind and the Corona

Kim¹,

Cairns²,

Robinson³

2008

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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

2014

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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.

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