Effects of a random spatial variation of the plasma density on the mode conversion in cold, unmagnetized, and stratified plasmas

Yu, Dae Jung; Kim, Kihong

doi:10.1063/1.4839175

Cited by 13 publications

(2 citation statements)

References 37 publications

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“…Alternative approaches were based on linear conversion mechanisms (Field 1956) of Langmuir or z-mode waves to electromagnetic radiation in plasmas with density gradients. Various aspects of such processes were considered by several authors (Melrose 1980;Cairns & Willes 2005;Yu & Kim 2013;Schleyer et al 2014); in particular, it was shown that such mechanisms may explain the radio emissions by Type III solar bursts (Thejappa et al 1993). The effectiveness of linear conversion mechanisms was also investigated using numerical methods (Cairns & Willes 2005;Kim et al 2008).…”

Section: Introductionmentioning

confidence: 99%

Efficiency of Electromagnetic Emission by Electrostatic Turbulence in Solar Wind and Coronal Plasmas with Density Inhomogeneities

Volokitin

Krafft

2020

ApJL

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We present a new method to semianalytically calculate the radiation efficiency of electromagnetic waves emitted at specific frequencies by electrostatic wave turbulence in solar wind and coronal plasmas with random density fluctuations. This method is applied to the case of electromagnetic emission radiated at the fundamental plasma frequency ω p by beam-driven Langmuir wave turbulence during Type III solar bursts. It is supposed that the main radiation mechanism is the linear conversion of electrostatic to electromagnetic waves on the background plasma density fluctuations, at constant frequency. The radiation efficiency (emissivity) of such a process is larger than that obtained in the framework of models where the low frequency density fluctuations and the corresponding ion sound waves are not external but produced by the electrostatic wave turbulence itself through nonlinear wavewave interactions. Results show that the radiation efficiency of Langmuir wave turbulence into electromagnetic emissions at ω p is nearly constant asymptotically, with the electromagnetic energy density growing linearly with time, and is proportional to the average level of density fluctuations. Comparisons with another analytical method developed by the authors and with space observations are satisfactory.

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Section: Introductionmentioning

confidence: 99%

Efficiency of Electromagnetic Emission by Electrostatic Turbulence in Solar Wind and Coronal Plasmas with Density Inhomogeneities

Volokitin

Krafft

2020

ApJL

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“…The mode conversion coefficient, which denotes the ratio of energy transformation from one mode to another mode, has a maximum value of about 0.5 for the linear or parabolic density profile. When the density profile is more complex, the scaling behavior is broken and the conversion coefficient can vary greatly [39][40][41][42]. While this scaling behavior is well-known in planar geometry, its behavior in cylindrical geometry was unknown until recently.…”

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confidence: 99%

Universal scaling behavior of resonant absorption

Doorsselaere

2019

Physics of Plasmas

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Mode conversion and resonant absorption are crucial mechanisms for wave transport and absorption. Scaling behavior of mode conversion or resonant absorption is well-known for electromagnetic and MHD waves in planar geometry. Our recent study showed that such a scaling behavior of resonant absorption could also exist for coronal loop oscillations with cylindrical geometry, but it was only tested for one density profile. Here we generalise our previous study on the scaling behavior of resonant absorption by considering multiple density profiles. Applying an invariant imbedding method to the ideal MHD wave equations, we show that the scaling behavior also exists for these density models. We thus generalise our earlier results and show that such a universal scaling exists in cylindrical geometry, too. Given these results and the earlier results in planar geometry, we formulate a hypothesis that a universal scaling behavior exists regardless of the type of mode conversion or resonant absorption.Mode conversion and resonant absorption can often occur in inhomogeneous plasmas such that two different waves are coupled with an exchange of energy. This mechanism has been considered as crucial for plasma heating and transport [4, 5, 14-17, 23, 28-31, 35]. The previous literature is mainly concerned with mode conversion between electromagnetic waves and electrostatic waves or between magnetoacoustic waves and Alfvén waves [e.g., 23, 44]. From theoretical and numerical studies, it is well established that a scaling behavior exists for the mode conversion coefficient in a planar geometry [8, 10-13, 17, 19, 20, 23, 27, 33, 38, 43]. The scaling parameter has in general a certain relation with the density gradient, wave vector, and wave frequency [see, e.g., 17, 19, 20, 33, 38]. The mode conversion coefficient, which denotes the ratio of energy transformation from one mode to another mode, has a maximum value of about 0.5 for the linear or parabolic density profile. When the density profile is more complex, the scaling behavior is broken and the conversion coefficient can vary greatly [39][40][41][42]. While this scaling behavior is well-known in planar geometry, its behavior in cylindrical geometry was unknown until recently. In our recent study, we considered kink modes in coronal loops, where the coronal loop was assumed as a infinitely long, straight, and axisymmetric cylinder with a radial inhomogeneity. We found that resonant absorption of the kink modes in the Alfvén resonance also show a certain scaling behavior. We called this behavior sub-universal therein. However, we only considered a sinusoidal density profile in the transitional layer [44]. Given that it was studied only for a particular density profile, it was unclear whether this behavior would be generally valid, as it is for planar geometry. This motivates the question if the results from our * djyu79@gmail.com previous paper can be generalised to other density profiles. If they can be generalised, it would imply that the scaling behavior is universal in inhomo...

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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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Effects of a random spatial variation of the plasma density on the mode conversion in cold, unmagnetized, and stratified plasmas

Cited by 13 publications

References 37 publications

Efficiency of Electromagnetic Emission by Electrostatic Turbulence in Solar Wind and Coronal Plasmas with Density Inhomogeneities

Efficiency of Electromagnetic Emission by Electrostatic Turbulence in Solar Wind and Coronal Plasmas with Density Inhomogeneities

Universal scaling behavior of resonant absorption

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

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